# What is the most dense object in the universe?

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Inspired by this answer to the question, Why is the Sun's density less than the inner planets?, what is the most dense object in the universe?

Let us define this as the largest observable density of a stable object, in order to exclude black holes which may have a very large (infinite) density at their centers or objects collapsing towards a black hole status.

If we restrict the definition in this way, then the answer should be the core of the most massive neutron star that we know about.

At present there are a couple of neutron stars with mass of about $2M_{odot}$ (Demorest et al. 2010; Antoniadis et al. 2013. Depending on the exact composition and equation of state at their centres these should have densities of around $2 imes 10^{18}$ kg/m$^{3}$ at their centers and average densities of $sim 10^{18}$ kg/m$^3$.

Note that these densities are around 3 times the density of a proton or neutron or 5-10 times the density of nuclei at zero pressure.

In principle, the density of a single electron is much higher.

## What is the densest thing on Earth?

Leaving aside the obvious political humor potential in your question, you did ask about the densest THING, not the densest material, on Earth. That would probably be a neutron. While many subatomic particles are considered points, and thus could be thought of as having an infinite density, the neutron has a measurable size (about 10 -15 m in diameter) as well as mass (about 1.7 x 10 -27 kg). Those numbers result in a density of about 10 18 kg/m 3 .

That value matches the estimated density of neutron stars, the densest objects known in the Universe. (A black hole's mass is concentrated in an immeasurable singularity). A single neutron is a smaller (MUCH smaller!) version of a neutron star, and the Earth is loaded with neutrons.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor

'For the sake of persons of . different types, scientific truth should be presented in different forms, and should be regarded as equally scientific, whether it appears in the robust form and the vivid coloring of a physical illustration, or in the tenuity and paleness of a symbolic expression.'

## The Birth of a Neutron Star

When stars with a mass 8 to 20 times greater than the Sun die, something very strange happens. The star explodes as a supernova, but its core still remains intact. If the core has more than 1.4 solar mass, it starts to collapse. The immense gravity presses the core into itself, along with all its subatomic particles, such as protons and electrons, combining to form an incredibly tight grid of plain neutrons.

This collapse takes a fraction of a second, but the change in the core&rsquos structure is drastic. The neutrons are so tightly packed that the core shrinks to a tiny sphere only 20 km in diameter, yet retaining its original mass. Remember that the Sun is 300,000 times more massive than the Earth. Now, imagine a star that is 500,000 times the mass of Earth crunched down to the size of a small city!

So, there you go! A neutron star is the most intensely dense object in all the universe. Of course, the argument can be made that a black hole is the most dense, but considering that a black hole is technically beyond the event horizon, it is neutron stars that get the top spot for the being the &lsquomost dense&rsquo.

Here are the top 5 reasons why neutron stars are the densest objects in the universe:

## 29.5 What Is the Universe Really Made Of?

Astronomy 29.5 What Is the Universe Really Made Of?

1 Science and the Universe: A Brief Tour

2 Observing the Sky: The Birth of Astronomy

6 Astronomical Instruments

7 Other Worlds: An Introduction to the Solar System

10 Earthlike Planets: Venus and Mars

13 Comets and Asteroids: Debris of the Solar System

14 Cosmic Samples and the Origin of the Solar System

15 The Sun: A Garden-Variety Star

16 The Sun: A Nuclear Powerhouse

18 The Stars: A Celestial Census

20 Between the Stars: Gas and Dust in Space

21 The Birth of Stars and the Discovery of Planets outside the Solar System

22 Stars from Adolescence to Old Age

24 Black Holes and Curved Spacetime

27 Active Galaxies, Quasars, and Supermassive Black Holes

28 The Evolution and Distribution of Galaxies

### Learning Objectives

By the end of this section, you will be able to:

• Specify what fraction of the density of the universe is contributed by stars and galaxies and how much ordinary matter (such as hydrogen, helium, and other elements we are familiar with here on Earth) makes up the overall density
• Describe how ideas about the contents of the universe have changed over the last 50 years
• Explain why it is so difficult to determine what dark matter really is
• Explain why dark matter helped galaxies form quickly in the early universe
• Summarize the evolution of the universe from the time the CMB was emitted to the present day

The model of the universe we described in the previous section is the simplest model that explains the observations. It assumes that general relativity is the correct theory of gravity throughout the universe. With this assumption, the model then accounts for the existence and structure of the CMB the abundances of the light elements deuterium, helium, and lithium and the acceleration of the expansion of the universe. All of the observations to date support the validity of the model, which is referred to as the standard (or concordance) model of cosmology.

Figure 29.21 and Table 29.2 summarize the current best estimates of the contents of the universe. Luminous matter in stars and galaxies and neutrinos contributes about 1% of the mass required to reach critical density. Another 4% is mainly in the form of hydrogen and helium in the space between stars and in intergalactic space. Dark matter accounts for about an additional 27% of the critical density. The mass equivalent of dark energy (according to E = mc 2 ) then supplies the remaining 68% of the critical density.

Object Density as a Percent of Critical Density
Luminous matter (stars, etc.) <1
Hydrogen and helium in interstellar and intergalactic space 4
Dark matter 27
Equivalent mass density of the dark energy 68

This table should shock you. What we are saying is that 95% of the stuff of the universe is either dark matter or dark energy—neither of which has ever been detected in a laboratory here on Earth. This whole textbook, which has focused on objects that emit electromagnetic radiation, has generally been ignoring 95% of what is out there. Who says there aren’t big mysteries yet to solve in science!

Figure 29.22 shows how our ideas of the composition of the universe have changed over just the past three decades. The fraction of the universe that we think is made of the same particles as astronomy students has been decreasing steadily.

### What Is Dark Matter?

Many astronomers find the situation we have described very satisfying. Several independent experiments now agree on the type of universe we live in and on the inventory of what it contains. We seem to be very close to having a cosmological model that explains nearly everything. Others are not yet ready to jump on the bandwagon. They say, “show me the 96% of the universe we can’t detect directly—for example, find me some dark matter!”

At first, astronomers thought that dark matter might be hidden in objects that appear dark because they emit no light (e.g., black holes) or that are too faint to be observed at large distances (e.g., planets or white dwarfs). However, these objects would be made of ordinary matter, and the deuterium abundance tells us that no more than 5% of the critical density consists of ordinary matter.

Another possible form that dark matter can take is some type of elementary particle that we have not yet detected here on Earth—a particle that has mass and exists in sufficient abundance to contribute 23% of the critical density. Some physics theories predict the existence of such particles. One class of these particles has been given the name WIMPs, which stands for weakly interacting massive particles . Since these particles do not participate in nuclear reactions leading to the production of deuterium, the deuterium abundance puts no limits on how many WIMPs might be in the universe. (A number of other exotic particles have also been suggested as prime constituents of dark matter, but we will confine our discussion to WIMPs as a useful example.)

If large numbers of WIMPs do exist, then some of them should be passing through our physics laboratories right now. The trick is to catch them. Since by definition they interact only weakly (infrequently) with other matter, the chances that they will have a measurable effect are small. We don’t know the mass of these particles, but various theories suggest that it might be a few to a few hundred times the mass of a proton. If WIMPs are 60 times the mass of a proton, there would be about 10 million of them passing through your outstretched hand every second—with absolutely no effect on you. If that seems too mind-boggling, bear in mind that neutrinos interact weakly with ordinary matter, and yet we were able to “catch” them eventually.

Despite the challenges, more than 30 experiments designed to detect WIMPS are in operation or in the planning stages. Predictions of how many times WIMPs might actually collide with the nucleus of an atom in the instrument designed to detect them are in the range of 1 event per year to 1 event per 1000 years per kilogram of detector. The detector must therefore be large. It must be shielded from radioactivity or other types of particles, such as neutrons, passing through it, and hence these detectors are placed in deep mines. The energy imparted to an atomic nucleus in the detector by collision with a WIMP will be small, and so the detector must be cooled to a very low temperature.

The WIMP detectors are made out of crystals of germanium, silicon, or xenon. The detectors are cooled to a few thousandths of a degree—very close to absolute zero. That means that the atoms in the detector are so cold that they are scarcely vibrating at all. If a dark matter particle collides with one of the atoms, it will cause the whole crystal to vibrate and the temperature therefore to increase ever so slightly. Some other interactions may generate a detectable flash of light.

A different kind of search for WIMPs is being conducted at the Large Hadron Collider (LHC) at CERN, Europe’s particle physics lab near Geneva, Switzerland. In this experiment, protons collide with enough energy potentially to produce WIMPs. The LHC detectors cannot detect the WIMPs directly, but if WIMPs are produced, they will pass through the detectors, carrying energy away with them. Experimenters will then add up all the energy that they detect as a result of the collisions of protons to determine if any energy is missing.

So far, none of these experiments has detected WIMPs. Will the newer experiments pay off? Or will scientists have to search for some other explanation for dark matter? Only time will tell (Figure 29.23).

### Dark Matter and the Formation of Galaxies

As elusive as dark matter may be in the current-day universe, galaxies could not have formed quickly without it. Galaxies grew from density fluctuations in the early universe, and some had already formed only about 400–500 million years after the Big Bang. The observations with WMAP, Planck, and other experiments give us information on the size of those density fluctuations. It turns out that the density variations we observe are too small to have formed galaxies so soon after the Big Bang. In the hot, early universe, energetic photons collided with hydrogen and helium, and kept them moving so rapidly that gravity was still not strong enough to cause the atoms to come together to form galaxies. How can we reconcile this with the fact that galaxies did form and are all around us?

Our instruments that measure the CMB give us information about density fluctuations only for ordinary matter, which interacts with radiation. Dark matter, as its name indicates, does not interact with photons at all. Dark matter could have had much greater variations in density and been able to come together to form gravitational “traps” that could then have begun to attract ordinary matter immediately after the universe became transparent. As ordinary matter became increasingly concentrated, it could have turned into galaxies quickly thanks to these dark matter traps.

For an analogy, imagine a boulevard with traffic lights every half mile or so. Suppose you are part of a motorcade of cars accompanied by police who lead you past each light, even if it is red. So, too, when the early universe was opaque, radiation interacted with ordinary matter, imparting energy to it and carrying it along, sweeping past the concentrations of dark matter. Now suppose the police leave the motorcade, which then encounters some red lights. The lights act as traffic traps approaching cars now have to stop, and so they bunch up. Likewise, after the early universe became transparent, ordinary matter interacted with radiation only occasionally and so could fall into the dark matter traps.

### The Universe in a Nutshell

In the previous sections of this chapter, we traced the evolution of the universe progressively further back in time. Astronomical discovery has followed this path historically, as new instruments and new techniques have allowed us to probe ever closer to the beginning of time. The rate of expansion of the universe was determined from measurements of nearby galaxies. Determinations of the abundances of deuterium, helium, and lithium based on nearby stars and galaxies were used to put limits on how much ordinary matter is in the universe. The motions of stars in galaxies and of galaxies within clusters of galaxies could only be explained if there were large quantities of dark matter. Measurements of supernovae that exploded when the universe was about half as old as it is now indicated that the rate of expansion of the universe has sped up since those explosions occurred. Observations of extremely faint galaxies show that galaxies had begun to form when the universe was only 400–500 million years old. And observations of the CMB confirmed early theories that the universe was initially very hot.

But all this moving further and further backward in time might have left you a bit dizzy. So now let’s instead show how the universe evolves as time moves forward.

Figure 29.24 summarizes the entire history of the observable universe from the beginning in a single diagram. The universe was very hot when it began to expand. We have fossil remnants of the very early universe in the form of neutrons, protons, electrons, and neutrinos, and the atomic nuclei that formed when the universe was 3–4 minutes old: deuterium, helium, and a small amount of lithium. Dark matter also remains, but we do not yet know what form it is in.

The universe gradually cooled when it was about 380,000 years old, and at a temperature of about 3000 K, electrons combined with protons to form hydrogen atoms. At this point, as we saw, the universe became transparent to light, and astronomers have detected the CMB emitted at this time. The universe still contained no stars or galaxies, and so it entered what astronomers call “the dark ages” (since stars were not lighting up the darkness). During the next several hundred million years, small fluctuations in the density of the dark matter grew, forming gravitational traps that concentrated the ordinary matter, which began to form galaxies about 400–500 million years after the Big Bang.

By the time the universe was about a billion years old, it had entered its own renaissance: it was again blazing with radiation, but this time from newly formed stars, star clusters, and small galaxies. Over the next several billion years, small galaxies merged to form the giants we see today. Clusters and superclusters of galaxies began to grow, and the universe eventually began to resemble what we see nearby.

During the next 20 years, astronomers plan to build giant new telescopes both in space and on the ground to explore even further back in time. In 2021, the James Webb Space Telescope, a 6.5-meter telescope that is the successor to the Hubble Space Telescope, will be launched and assembled in space. The predictions are that with this powerful instrument (see Figure 29.1) we should be able to look back far enough to analyze in detail the formation of the first galaxies.

## The densest objects in the Universe

When a massive star explodes, not all the material is ejected into space. Some of it collapses into an extremely compact object known as a neutron star, inside which gravitational forces crush protons and electrons together, turning them into particles known as neutrons.

A neutron star contains a few solar masses of material squeezed into a radius of only 20 km. This means the matter is so compressed that a thimble full of it would weigh millions of tonnes on Earth. Fast-spinning neutron stars, whose radio emissions seem to pulse on and off, are called pulsars.

Beyond the mass limit of a neutron star - about three solar masses - gravity becomes overwhelming and collapses the star even further, creating a black hole. These are perhaps the strangest objects in the Universe because nothing, not even light, can escape from inside a black hole. So, the presence of a black hole can only be inferred by its effect on surrounding celestial objects and other interstellar material.

Virtually all types of compact objects are significant sources of high-energy emission because of the enormous gravitational fields they tend to generate. Gravitational fields can accelerate particles in the vicinity to extreme velocities, which then emit gamma rays and X-rays.

Integral will capture images of the high-energy emission from such compact objects with unprecedented detail, allowing astronomers a clearer look than ever before at these enigmatic objects.

## Astronomers Detect the Most Massive Neutron Star Yet

The star's white dwarf companion helped scientists measure its extraordinary mass.

Astronomers have discovered the most massive example yet of the dead stars known as neutron stars, one almost too massive to exist, a new study finds.

Neutron stars, like black holes, are corpses of stars that died in catastrophic explosions known as supernovas. When a star goes supernova, the core of its remains collapses under the strength of its own gravitational pull. If this remnant is massive enough, it may form a black hole, which has gravity so powerful that not even light can escape. A less massive core will form a neutron star, so named because its gravity is strong enough to crush protons together with electrons to form neutrons.

"Neutron stars are as mysterious as they are fascinating," study lead author Thankful Cromartie at the University of Virginia and the National Radio Astronomy Observatory, both in Charlottesville, Virginia, said in a statement. "These city-sized objects are essentially ginormous atomic nuclei."

Neutron stars are usually small, with diameters of about 12 miles (19 kilometers) or so, but they are extraordinarily dense. A neutron star's mass is often about the same as that of the sun a sugar-cube's worth of neutron-star material has a mass of about 100 million tons, or about the same as the entire human population, according to the statement. This makes neutron stars the universe's densest objects besides black holes.

Although scientists have studied neutron stars for decades, many of their mysteries remain unsolved. For example, do the incredible pressures found within neutron stars break neutrons down into soups of still tinier subatomic particles known as quarks? What is the tipping point when gravity wins out over matter and forms a black hole?

"These stars are very exotic," study co-author Maura McLaughlin at West Virginia University in Morgantown said in a separate statement. "We don't know what they're made of and one really important question is, 'How massive can you make one of these stars?' It has implications for very exotic material that we simply can't create in a laboratory on Earth."

The newly measured neutron star, called J0740+6620, lies about 4,600 light-years from Earth. It packs 2.14 times the mass of the sun into a sphere only about 15 miles (25 km) in diameter. That approaches the theoretical limits of how massive and compact a single object can become without crushing itself down under the force of its own gravitational pull into a black hole.

"Neutron stars have this tipping point where their interior densities get so extreme that the force of gravity overwhelms even the ability of neutrons to resist further collapse," study co-author Scott Ransom, an astronomer at the National Radio Astronomy Observatory, said in a statement. "Each 'most massive' neutron star we find brings us closer to identifying that tipping point and helping us to understand the physics of matter at these mind-boggling densities."

J0740+6620 is a kind of rotating neutron star known as a pulsar. Pulsars emit twin beams of radio waves from their magnetic poles, flashing like lighthouse beacons &mdash hence their name, which is short for "pulsating star." Specifically, J0740+6620 is a type of pulsar known as a millisecond pulsar, which rapidly spin hundreds of revolutions per second.

Astronomers measured the mass of this pulsar through a phenomenon known as "Shapiro Delay." In essence, the gravity from the pulsar's white dwarf companion &mdash a small, dense star that co-orbits the neutron star &mdash warps the fabric of space and time around it to a degree proportionate to the white dwarf's mass. These distortions in space-time delay pulses from the pulsar by tens of millionths of a second when it passes behind the white dwarf from the viewpoint of Earth. Scientists can measure these delays to calculate the mass of the white dwarf. By analyzing the way in which the pulsar and white dwarf orbit one another, the researchers can then estimate the mass of the pulsar.

This discovery was a serendipitous result made during routine observations taken as part of a search for invisible ripples in space-time known as gravitational waves using the Green Bank Telescope in West Virginia, McLaughlin said.

"At Green Bank, we're trying to detect gravitational waves from pulsars," he said. "In order to do that, we need to observe lots of millisecond pulsars."

The scientists detailed their findings Sept. 16 in the journal Nature Astronomy.

## The Largest Structures In The Universe May Not Actually Exist

This visualization of the Laniakea supercluster, which represents a collection of more than 100,000 . [+] estimated galaxies spanning a volume of over 100 million light-years, shows the distribution of dark matter (shadowy purple) and individual galaxies (bright orange/yellow) together. Despite the relatively recent identification of Laniakea as the supercluster which contains the Milky Way and much more, it's not a gravitationally bound structure and will not hold together as the Universe continues to expand.

Tsaghkyan / Wikimedia Commons

In theory, the Universe should be the same, on average, everywhere.

A simulation of the large-scale structure of the Universe. While, on small scales, various regions . [+] are dense and massive enough to correspond to star clusters, galaxies, and galaxy clusters, while others correspond to cosmic voids, on larger scales, every location is largely similar to every other location.

On the largest scales, it shouldn’t matter which direction you observe.

This image shows a map of the full sky and the X-ray clusters identified to measure the expansion of . [+] the Universe in a direction-dependent way, along with four X-ray clusters in detail imaged by NASA's Chandra X-ray observatory. Although the results suggest the Universe's expansion may not be isotropic, or the same in all directions, the data is far from clear-cut, and the anisotropic interpretation was heavily criticized.

NASA/CXC/Univ. of Bonn/K. Migkas et al.

Nor should it matter which location you’re examining.

In modern cosmology, a large-scale web of dark matter and normal matter permeates the Universe. On . [+] the scales of individual galaxies and smaller, the structures formed by matter are highly non-linear, with densities that depart from the average density by enormous amounts. On very large scales, however, the density of any region of space is very close to the average density: to about 99.99% accuracy.

WESTERN WASHINGTON UNIVERSITY

We expect isotropy and homogeneity, with physical consequences if they’re violated.

The early Universe was full of matter and radiation, and was so hot and dense that the quarks and . [+] gluons present didn't form into individual protons and neutrons, but remained in a quark-gluon plasma. This primordial soup consisted of particles, antiparticles, and radiation, and although was in a lower entropy state than our modern Universe, there was still plenty of entropy.

RHIC COLLABORATION, BROOKHAVEN

### 29 Intelligent Alien Civilizations May Have Already Spotted Us, Say Scientists

Initially, the Big Bang simultaneously occurred everywhere.

The full suite of what's present today in the Universe owes its origins to the hot Big Bang. More . [+] fundamentally, the Universe we have today can only come about because of the properties of spacetime and the laws of physics. Without them, we cannot have existence in any form.

All locations possessed equivalent temperatures and densities.

As our satellites have improved in their capabilities, they've probes smaller scales, more frequency . [+] bands, and smaller temperature differences in the cosmic microwave background. The temperature imperfections help teach us what the Universe is made of and how it evolved, painting a picture that requires dark matter to make sense.

NASA/ESA AND THE COBE, WMAP AND PLANCK TEAMS PLANCK 2018 RESULTS. VI. COSMOLOGICAL PARAMETERS PLANCK COLLABORATION (2018)

Only tiny, 1-part-in-30,000 imperfections get superimposed atop them.

The large-scale structure of the Universe changes over time, as tiny imperfections grow to form the . [+] first stars and galaxies, then merge together to form the large, modern galaxies we see today. Looking to great distances reveals a younger Universe, similar to how our local region was in the past. The temperature fluctuations in the CMB, as well as the clustering properties of galaxies throughout time, provide a unique method of measuring the Universe’s expansion history.

Chris Blake and Sam Moorfield

Those imperfections then evolved gravitationally, limited by our physical laws.

This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, . [+] represents billions of years of gravitational growth in a dark matter-rich Universe. Note that filaments and rich clusters, which form at the intersection of filaments, arise primarily due to dark matter normal matter plays only a minor role.

Ralf Kähler and Tom Abel (KIPAC)/Oliver Hahn

Tremendous cosmological structures formed: stars, galaxies, and the great cosmic web.

A map of more than one million galaxies in the Universe, where each dot is its own galaxy. On these . [+] large scales, it becomes clear that the clustering patterns we see are important on small cosmic scales, but as we look to larger and larger scales, the Universe appears more uniform.

Daniel Eisenstein and the SDSS-III collaboration

We expect a structural size limit:

The 3D reconstruction of 120,000 galaxies and their clustering properties, inferred from their . [+] redshift and large-scale structure formation. The left, black-and-white image is the raw data, the green dots show the reconstructed 3D positions of those same galaxies.

Jeremy Tinker and the SDSS-III collaboration

Anything larger wouldn’t have sufficient time to form.

Both simulations (red) and galaxy surveys (blue/purple) display the same large-scale clustering . [+] patterns as one another, even when you look at the mathematical details. If dark matter weren't present, a lot of this structure would not only differ in detail, but would be washed out of existence galaxies would be rare and filled with almost exclusively light elements.

GERARD LEMSON AND THE VIRGO CONSORTIUM

The warm-hot intergalactic medium (WHIM) has been seen along incredibly overdense regions, like the . [+] Sculptor wall, illustrated above. These walls are enormous, but no larger than 1.4 billion light-years, at least as have been confirmed to exist. Still, it's conceivable that there are still surprises out there in the Universe.

Spectrum: NASA/CXC/Univ. of California Irvine/T. Fang. Illustration: CXC/M. Weiss

Similarly, great cosmic voids exist between them.

A region of space devoid of matter in our galaxy reveals the Universe beyond, where every point is a . [+] distant galaxy. The cluster/void structure can be seen very clearly, demonstrating that our Universe is not of exactly uniform density on all scales. Everywhere we look, however, we still find 'something' in the Universe.

These largest structures approach, but don’t significantly exceed, the expected cosmic limits.

This figure shows the relative attractive and repulsive effects of overdense and underdense regions . [+] on the Milky Way. Note that, despite the large number of galaxies clumped and clustered nearby, there are also large regions that have extremely few galaxies: cosmic voids. While we have a few substantial ones nearby, there are even larger and lower-density voids found in the distant Universe, but nothing defying our cosmic expectations.

Yehuda Hoffman, Daniel Pomarède, R. Brent Tully, and Hélène Courtois, Nature Astronomy 1, 0036 (2017)

But two classes of structures threaten this picture.

Some quasar groupings appear to be clustered and/or aligned on larger cosmic scales than are . [+] predicted. The largest of them, known as the Huge Large Quasar Group (Huge-LQG), consists of 73 quasars spanning up to 5-6 billion light-years, but may only be what's known as a pseudo-structure.

Three separate large quasar groupings are clustered across too-large cosmic scales.

Here, two different large quasar groupings are shown: the Clowes-Campusano LQG in red and the . [+] Huge-LQG in black. Just two degrees away, another LQG has been found as well. however, whether these are just unrelated quasar locations or a true larger-than-expected set of structures remains unresolved.

R. G. Clowes/University of Central Lancashire SDSS

Similarly galaxy groups from gamma-ray burst mapping surpass these limits.

NASA's Fermi Satellite has constructed the highest resolution, high-energy map of the Universe ever . [+] created. Without space-based observatories such as this one, we could never learn all that we have about the Universe, nor could we even accurately measure the gamma-ray sky. Some gamma-ray bursts appear to be clustered in a way that may indicate larger-than-expected cosmic structures.

NASA/DOE/Fermi LAT Collaboration

If real, these structures defy our present cosmic understanding.

This illustration of the large GRB ring, and the inferred underlying large-scale structure, shows . [+] what might be responsible for the pattern we've observed. However, this may not be a true structure, but only a pseudo-structure, and we may be fooling ourselves by believing this extends across many billions of light-years of space.

Pablo Carlos Budassi/Wikimedia.org

However, they may be purely phantasmal.

This illustration of the most distant gamma-ray burst ever detected, GRB 090423, is thought to be . [+] typical of most fast gamma-ray bursts. However, whether the multiple gamma-ray bursts we've seen are good tracers of the underlying large-scale structure or not remains a debated topic.

These signals may emerge from underlying random noise, with statistics incorrectly “discovering” non-existent patterns.

Combination image of quasar RX J1131 (center) taken via NASA’s Chandra X-ray Observatory and the . [+] Hubble Space Telescope. Microlensing events associated with this quasar provide evidence for some

2,000 rogue/orphan planets populating the interstellar space around this quasar's core, making this the most distant location known that contains planets. While other quasars and structures can be found nearby, we can tell that this object isn't part of a structure that's larger than the expected cosmic limits.

NASA/CXC/Univ of Michigan/R.C.Reis et al

Only superior data, sufficiently mapping out our Universe, will decide.

The Hubble Ultra-Deep Field, shown in blue, is currently the largest, deepest long-exposure campaign . [+] undertaken by humanity. For the same amount of observing time, the Nancy Grace Roman Telescope will be able to image the orange area to the exact same depth, revealing over 100 times as many objects as are present in the comparable Hubble image. We should finally be able to test whether these quasar and gamma-ray burst clusterings are real structures, or just pseudo-structures.

NASA, ESA, and A. Koekemoer (STScI) Acknowledgement: Digitized Sky Survey

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less smile more.

## The Inflationary Hypothesis

Some physicists suggested that these fundamental characteristics of the cosmos—its flatness and uniformity—can be explained if shortly after the Big Bang (and before the emission of the CMB), the universe experienced a sudden increase in size. A model universe in which this rapid, early expansion occurs is called an inflationary universe. The inflationary universe is identical to the Big Bang universe for all time after the first 10 –30 second. Prior to that, the model suggests that there was a brief period of extraordinarily rapid expansion or inflation, during which the scale of the universe increased by a factor of about 10 50 times more than predicted by standard Big Bang models (Figure 1).

Figure 1. Expansion of the Universe: This graph shows how the scale factor of the observable universe changes with time for the standard Big Bang model (red line) and for the inflationary model (blue line). (Note that the time scale at the bottom is extremely compressed.) During inflation, regions that were very small and in contact with each other are suddenly blown up to be much larger and outside each other’s horizon distance. The two models are the same for all times after 10–30 second.

Prior to (and during) inflation, all the parts of the universe that we can now see were so small and close to each other that they could exchange information, that is, the horizon distance included all of the universe that we can now observe. Before (and during) inflation, there was adequate time for the observable universe to homogenize itself and come to the same temperature. Then, inflation expanded those regions tremendously, so that many parts of the universe are now beyond each other’s horizon.

Another appeal of the inflationary model is its prediction that the density of the universe should be exactly equal to the critical density. To see why this is so, remember that curvature of spacetime is intimately linked to the density of matter. If the universe began with some curvature of its spacetime, one analogy for it might be the skin of a balloon. The period of inflation was equivalent to blowing up the balloon to a tremendous size. The universe became so big that from our vantage point, no curvature should be visible (Figure 2). In the same way, Earth’s surface is so big that it looks flat to us no matter where we are. Calculations show that a universe with no curvature is one that is at critical density. Universes with densities either higher or lower than the critical density would show marked curvature. But we saw that the observations of the CMB in The Cosmic Microwave Background, which show that the universe has critical density, rule out the possibility that space is significantly curved.

Figure 2. Analogy for Inflation: During a period of rapid inflation, a curved balloon grows so large that to any local observer it looks flat. The inset shows the geometry from the ant’s point of view.

## What is the most dense object in the universe? - Astronomy

Well, in general, I'm trying to reconcile two concepts. So, basically, my question is what does the edge of the universe look like compared to relatively closer "areas" of space? Is it more dense, or less dense? The maps of the universe I have seen show that the density of galaxies drops off at very large distances, but we also know something else--as we look further away we are looking "longer ago" and closer to the big bang--and those images look more dense. So is this to say that more distant galaxies, or galaxy-clusters are "more dense" than closer galaxies and galaxy clusters. but there are just fewer galaxies and galaxy-clusters that we can see very far away? And not all slivers of space that we map show the most distant galaxies? Or does this simply have to do with the interdepedency of light and time? Meaning that we see something different than we mathematically extrapolate to map? I'm just curious as to how these facts are reconciled. If you don't underst! and the question, just notify me that I'm not being clear and I'll rephrase it. But to be honest, I'm simply trying to reconcile the pictures on pages 22,37,40,and 72 of Hawking's Universe in a Nutshell.

The short answer is that it's harder to see things that are farther away. So while we can see almost all the galaxies nearby, we can only see the very brightest ones far away. This effect overwhelms everything else, and is responsible for the density of galaxies in those maps dropping off at large distances. So if you look at one of those maps, you can imagine that there are actually many more galaxies on the outskirts, but we just can't see them.

What if you weren't limited by this effect? What if you could see *everything* out to the edge of the observable universe? If you looked out to the edge of the universe, you'd see the universe at a time when it was very young. You would see the pieces of what would eventually become galaxies. These would appear more densely spaced together than galaxies are today because there are more of them (they haven't had the chance to merge together to reduce their number). There's also the fact that the universe was physically smaller at this early time, because it hadn't had as much time to expand. You might expect that this would translate into an even higher apparent density, but actually it doesn't. The "mini-galaxies" are projected on the sky in such a way that this doesn't happen. (An apparent distance projected on the sky turns out to be different from what you would expect from Euclidean geometry.)

The answer on this page is somewhat confusing. Probably by use of the word 'apparent'. I'm not sure what the difference between 'apparent density' and just regular density is.

If we define the "density of galaxies" as the number of galaxies per unit volume, then the density does in fact decrease as time goes on (it was greater in the past than it is now). But the question specifically related to what we observe in galaxy catalogs. Can you tell that the density of the universe was greater in the past than it is now by looking at the distribution of objects on the sky? No. Why not?

Imagine that you're looking at a very distant galaxy in one part of the sky, and then compare it to another very distant galaxy in another part of the sky. The angular separation of those two galaxies can be very large. So you could say that it "looks" like they're billions of light-years apart. But yet in the very distant past, when the universe was much much smaller than it is now, they were physically very close together. So you can't really measure the density of the universe at that early time by counting up galaxies and dividing by the volume they appear to occupy just as you would in a universe that wasn't expanding. The expansion of the universe means that objects that were very close together at the time they emitted the light that we're now seeing are spread out over the sky in a way that wouldn't happen in a universe that wasn't expanding.

#### Christopher Springob

Chris studies the large scale structure of the universe using the peculiar velocities of galaxies. He got his PhD from Cornell in 2005, and is now a Research Assistant Professor at the University of Western Australia.

## What is the most dense object in the universe? - Astronomy

Well, in general, I'm trying to reconcile two concepts. So, basically, my question is what does the edge of the universe look like compared to relatively closer "areas" of space? Is it more dense, or less dense? The maps of the universe I have seen show that the density of galaxies drops off at very large distances, but we also know something else--as we look further away we are looking "longer ago" and closer to the big bang--and those images look more dense. So is this to say that more distant galaxies, or galaxy-clusters are "more dense" than closer galaxies and galaxy clusters. but there are just fewer galaxies and galaxy-clusters that we can see very far away? And not all slivers of space that we map show the most distant galaxies? Or does this simply have to do with the interdepedency of light and time? Meaning that we see something different than we mathematically extrapolate to map? I'm just curious as to how these facts are reconciled. If you don't underst! and the question, just notify me that I'm not being clear and I'll rephrase it. But to be honest, I'm simply trying to reconcile the pictures on pages 22,37,40,and 72 of Hawking's Universe in a Nutshell.

The short answer is that it's harder to see things that are farther away. So while we can see almost all the galaxies nearby, we can only see the very brightest ones far away. This effect overwhelms everything else, and is responsible for the density of galaxies in those maps dropping off at large distances. So if you look at one of those maps, you can imagine that there are actually many more galaxies on the outskirts, but we just can't see them.

What if you weren't limited by this effect? What if you could see *everything* out to the edge of the observable universe? If you looked out to the edge of the universe, you'd see the universe at a time when it was very young. You would see the pieces of what would eventually become galaxies. These would appear more densely spaced together than galaxies are today because there are more of them (they haven't had the chance to merge together to reduce their number). There's also the fact that the universe was physically smaller at this early time, because it hadn't had as much time to expand. You might expect that this would translate into an even higher apparent density, but actually it doesn't. The "mini-galaxies" are projected on the sky in such a way that this doesn't happen. (An apparent distance projected on the sky turns out to be different from what you would expect from Euclidean geometry.)

The answer on this page is somewhat confusing. Probably by use of the word 'apparent'. I'm not sure what the difference between 'apparent density' and just regular density is.

If we define the "density of galaxies" as the number of galaxies per unit volume, then the density does in fact decrease as time goes on (it was greater in the past than it is now). But the question specifically related to what we observe in galaxy catalogs. Can you tell that the density of the universe was greater in the past than it is now by looking at the distribution of objects on the sky? No. Why not?

Imagine that you're looking at a very distant galaxy in one part of the sky, and then compare it to another very distant galaxy in another part of the sky. The angular separation of those two galaxies can be very large. So you could say that it "looks" like they're billions of light-years apart. But yet in the very distant past, when the universe was much much smaller than it is now, they were physically very close together. So you can't really measure the density of the universe at that early time by counting up galaxies and dividing by the volume they appear to occupy just as you would in a universe that wasn't expanding. The expansion of the universe means that objects that were very close together at the time they emitted the light that we're now seeing are spread out over the sky in a way that wouldn't happen in a universe that wasn't expanding.