Thanks to the presence of a natural "zoom lens" in space, this is a close-up look at the brightest distant "magnified" galaxy in the universe known to date. Credit: NASA, ESA, J. Rigby (NASA Goddard Space Flight Center), K. Sharon (Kavli Institute for Cosmological Physics, University of Chicago), and M. Gladders and E. Wuyts (University of Chicago)
[/caption]
Less than a year ago, the Hubble Space Telescope’s Wide Field Camera 3 captured an amazing image – a giant lensed galaxy arc. Gravitational lensing produces a natural “zoom” to observations and this is a look at one of the brightest distant galaxies so far known. Located some 10 billion light years away, the galaxy has been magnified as a nearly 90-degree arc of light against the galaxy cluster RCS2 032727-132623 – which is only half the distance. In this unusual case, the background galaxy is over three times brighter than typically lensed galaxies… and a unique look back in time as to what a powerful star-forming galaxy looked like when the Universe was only about one third its present age.
A team of astronomers led by Jane Rigby of NASA’s Goddard Space Flight Center in Greenbelt, Maryland are the parties responsible for this incredible look back into time. It is one of the most detailed looks at an incredibly distant object to date and their results have been accepted for publication in The Astrophysical Journal, in a paper led by Keren Sharon of the Kavli Institute for Cosmological Physics at the University of Chicago. Professor Michael Gladders and graduate student Eva Wuyts of the University of Chicago were also key team members.
“The presence of the lens helps show how galaxies evolved from 10 billion years ago to today. While nearby galaxies are fully mature and are at the tail end of their star-formation histories, distant galaxies tell us about the universe’s formative years. The light from those early events is just now arriving at Earth.” says the team. “Very distant galaxies are not only faint but also appear small on the sky. Astronomers would like to see how star formation progressed deep within these galaxies. Such details would be beyond the reach of Hubble’s vision were it not for the magnification made possible by gravity in the intervening lens region.”
This graphic shows a reconstruction (at lower left) of the brightest galaxy whose image has been distorted by the gravity of a distant galaxy cluster. The small rectangle in the center shows the location of the background galaxy on the sky if the intervening galaxy cluster were not there. The rounded outlines show distinct, distorted images of the background galaxy resulting from lensing by the mass in the cluster. The image at lower left is a reconstruction of what the lensed galaxy would look like in the absence of the cluster, based on a model of the cluster's mass distribution derived from studying the distorted galaxy images. Illustration Credit: NASA, ESA, and Z. Levay (STScI) Science Credit: NASA, ESA, J. Rigby (NASA Goddard Space Flight Center), K. Sharon (Kavli Institute for Cosmological Physics, University of Chicago), and M. Gladders and E. Wuyts (University of Chicago)
But the Hubble isn’t the only eye on the sky examining this phenomenon. A little over 10 years ago a team of astronomers using the Very Large Telescope in Chile also measured and examined the arc and reported the distant galaxy seems to be more than three times brighter than those previously discovered. However, there’s more to the picture than meets the eye. Original images show the magnified galaxy as hugely distorted and it shows itself more than once in the foreground lensing cluster. The challenge was to create a image that was “true to life” and thanks to Hubble’s resolution capabilities, the team was able to remove the distortions from the equation. In this image they found several incredibly bright star-forming regions and through the use of spectroscopy, they hope to better understand them.
The LABOCA camera on the ESO-operated 12-metre Atacama Pathfinder Experiment (APEX) telescope reveals distant galaxies undergoing the most intense type of star formation activity known, called a starburst. This image shows these distant galaxies, found in a region of sky known as the Extended Chandra Deep Field South, in the constellation of Fornax (The Furnace). The galaxies seen by LABOCA are shown in red, overlaid on an infrared view of the region as seen by the IRAC camera on the Spitzer Space Telescope. Credit: ESO, APEX (MPIfR/ESO/OSO), A. Weiss et al., NASA Spitzer Science Center
[/caption]
Located on the Chajnantor plateau in the foothills of the Chilean Andes, ESO’s APEX telescope has been busy looking into deep, deep space. Recently a group of astronomers released their findings regarding massive galaxies in connection with extreme times of star formation in the early Universe. What they found was a sharp cut-off point in stellar creation, leaving “massive – but passive – galaxies” filled with mature stars. What could cause such a scenario? Try the materialization of a supermassive black hole…
By integrating data taken with the LABOCA camera on the ESO-operated 12-metre Atacama Pathfinder Experiment (APEX) telescope with measurements made with ESO’s Very Large Telescope, NASA’s Spitzer Space Telescope and other facilities, astronomers were able to observe the relationship of bright, distant galaxies where they form into clusters. They found that the density of the population plays a major role – the tighter the grouping, the more massive the dark matter halo. These findings are the considered the most accurate made so far for this galaxy type.
Located about 10 billion light years away, these submillimetre galaxies were once home to starburst events – a time of intense formation. By obtaining estimations of dark matter halos and combining that information with computer modeling, scientists are able to hypothesize how the halos expanding with time. Eventually these once active galaxies settled down to form giant ellipticals – the most massive type known.
“This is the first time that we’ve been able to show this clear link between the most energetic starbursting galaxies in the early Universe, and the most massive galaxies in the present day,” says team leader Ryan Hickox of Dartmouth College, USA and Durham University, UK.
However, that’s not all the new observations have uncovered. Right now there’s speculation the starburst activity may have only lasted around 100 million years. While this is a very short period of cosmological time, this massive galactic function was once capable of producing double the amount of stars. Why it should end so suddenly is a puzzle that astronomers are eager to understand.
“We know that massive elliptical galaxies stopped producing stars rather suddenly a long time ago, and are now passive. And scientists are wondering what could possibly be powerful enough to shut down an entire galaxy’s starburst,” says team member Julie Wardlow of the University of California at Irvine, USA and Durham University, UK.
Right now the team’s findings are offering up a new solution. Perhaps at one point in cosmic history, starburst galaxies may have clustered together similar to quasars… locating themselves in the same dark matter halos. As one of the most kinetic forces in our Universe, quasars release intense radiation which is reasoned to be fostered by central black holes. This new evidence suggests intense starburst activity also empowers the quasar by supplying copious amounts of material to the black hole. In response, the quasar then releases a surge of energy which could eradicate the galaxy’s leftover gases. Without this elemental fuel, stars can no longer form and the galaxy growth comes to a halt.
“In short, the galaxies’ glory days of intense star formation also doom them by feeding the giant black hole at their centre, which then rapidly blows away or destroys the star-forming clouds,” explains team member David Alexander from Durham University, UK.
A visible light image of the FGC 1287 group of galaxies in Abell 1367. This is based on a composite of images taken from the Sloan Digital Sky Survey through three colour filters. The white contours show the neutral hydrogen distribution. The huge gas tail emanates from the edge on spiral galaxy FGC 1287. Two other members of the group have associated neutral hydrogen here marked by contour lines.
[/caption]
When it comes to understanding how galaxies behave both inside and outside of galaxy clusters, it would seem that we still have quite a lot to learn. Tom Scott from the Instituto de Astrofisica de Andalucia in Granada, Spain, and a group of international astronomers have been busy with the Expanded Very Large Array (EVLA) of the National Radio Astronomy Observatory (NRAO) in the USA, checking out an assortment of galaxies associated with galaxy cluster Abell 1367. What they have found is unexpectedly long one-sided gaseous tails in two sets of galaxies… the longest of their type ever observed.
Located in the constellation of Leo and about 300 million light years away, galaxies CGCG 097-026 and FGC1287 are displaying gaseous tail structures that may rearrange thinking on how stripping of materials behaves. Current thinking has hot gases trapped within the galaxy cluster’s gravitational field – with incoming galaxies being depleted of their cold hydrogen gases when captured by the gravitational influence. Through this impact, galaxies added to the cluster generally tend to lose their star-forming abilities and begin to quickly age. Astronomers assume this is why less aggressive galaxy structures tend to be found in lower density environments. However, thanks to Scott’s research, astronomers might be able to assume that galaxies can be robbed of their gases before entering a clustered environment.
“When we looked at the data, we were amazed to see these tail structures” says Tom Scott. “The projected lengths of the gaseous tails are 9 to 10 times that of the size of the parent galaxies, i.e., 520,000 and 815,000 light years respectively. In both cases the amount of cold hydrogen gas in the tails is approximately the same as that remaining in the galaxy’s disk. In other words, these galaxies have already left behind half of their fuel for star formation before entering the sphere of influence of the cluster.”
As stated, the commonly accepted theory for gaseous tail structures is interaction with the hot, gaseous medium located within the cluster’s influence – a process known as ram-pressure stripping. But this case is different. Galaxies CGCG 097-026 and FGC1287 aren’t being perturbed by the nearby cluster just yet… But they are still displaying long tails of material.
“We considered the various physical processes proposed by theorists in the past to describe gas removal from galaxies, but no one seems to be able to explain our observations” says Luca Cortese, researcher at ESO-Garching, Germany, and co-author of this work. “Whereas in the case of CGCG97-026, the gravitational interaction between the various members of the group could explain what we see, FGC1287 is completely different from any case we have seen before.”
Right now, ram-pressure stripping isn’t the answer – and gravitational interactions don’t seem to fit the picture, either. It’s leaving scientists at a loss to explain these long tails and lack of stellar disturbance.
“Although the mechanism responsible for this extraordinary gas tail remains to be determined, our discovery highlights how much there still is to learn about environmental effects in galaxy groups” says team member Elias Brinks, a scientist at the University of Hertfordshire. “This discovery might open a new chapter in our understanding of environmental effects on galaxy evolution.”
This image of galaxy cluster MACS J1206.2-0847 is part of a broad survey with NASA's Hubble Space Telescope. Credit: NASA, ESA, M. Postman (STScI), and the CLASH Team
[/caption]
Way off in the constellation of Virgo, galaxy cluster MACS J1206.2-0847 -or MACS 1206 for short – is making news as the forerunner of a brand new Hubble Space Telescope survey. What’s new for the aging telescope? Now astronomers are able to assemble a highly detailed dark matter map… one that involves more galaxy clusters than ever before.
These “dark matter” maps are proving their worth by allowing astronomers to test some theories. In this case, it’s some unusual findings which suggest dark matter is more densely packed inside galaxy clusters than some models predict. If this holds true, it may point to evidence that galaxy clusters pulled together sooner than once predicted. The multiwavelength survey, called the Cluster Lensing And Supernova survey with Hubble (CLASH), takes an unprecedented look at the distribution of dark matter in 25 massive clusters of galaxies.
“The era when the first clusters formed is not precisely known, but is estimated to be at least 9 billion years ago and possibly as far back as 12 billion years ago.” says the Hubble team. “If most of the clusters in the CLASH survey are found to have excessively high accumulations of dark matter in their central cores, then it may yield new clues to the early stages in the origin of structure in the universe.”
To date, the CLASH team has finished their observations of six of the 25 clusters. Of these, MACS 1206 has a distance of about 4.5 billion light-years and was photographed with Hubble’s Advanced Camera for Surveys and the Wide Field Camera 3 in April 2011 through July 2011. What are the strange shapes? These “distortions” are where the light is bent by the extreme gravitation of dark matter.
“Lensing effects can also produce multiple images of the same distant object, as evident in this Hubble picture. In particular, the apparent numbers and shapes of distant galaxies far beyond a galaxy cluster become distorted as the light passes through, yielding a visible measurement of how much mass is in the intervening cluster and how it is distributed.” says the team. “The substantial lensing distortions seen are proof that the dominant component of clusters is dark matter. The distortions would be far weaker if the clusters’ gravity came only from the visible galaxies in the clusters.”
A cluster of galaxies as seen from the Hubble Space Telescope
[/caption]
Hey. We’re all aware of Einstein’s theories and how gravity affects light. We know it was proved during a total solar eclipse, but what we’ve never realized in observational astronomy is that light just might get bent by other gravitational influences. If it can happen from something as small as a star, then what might occur if you had a huge group of stars? Like a galaxy… Or a group of galaxies!
What’s new in the world of light? Astrophysicists at the Dark Cosmology Centre at the Niels Bohr Institute have now gone around the bend and came up with a method of measuring how outgoing light is affected by the gravity of galaxy clusters. Not only does each individual star and each individual galaxy possess its own gravity, but a galaxy group is held together by gravitational attraction as well. Sure, it stands to reason that gravity is affecting what we see – but there’s even more to it. Redshift…
“It is really wonderful. We live in an era with the technological ability to actually measure such phenomena as cosmological gravitational redshift”, says astrophysicist Radek Wojtak, Dark Cosmology Centre under the Niels Bohr Institute at the University of Copenhagen.
Together with team members Steen Hansen and Jens Hjorth, Wojtak has been collecting light data and measurements from 8,000 galaxy clusters. Their studies have included calculations from mid-placed members to calibrations on those that reside at the periphery.
“We could measure small differences in the redshift of the galaxies and see that the light from galaxies in the middle of a cluster had to ‘crawl’ out through the gravitational field, while it was easier for the light from the outlying galaxies to emerge”, explains Radek Wojtak.
Until now, the gravitational redshift has only been tested with experiments and observations in relation to distances her on Earth and in relation to the solar system. With the new research the theory has been tested on a cosmological scale for the first time by analyzing galaxies in galaxy clusters in the distant universe. It is a grotesquely large scale, which is a factor 1,022 times greater (ten thousand billion billion times larger than the laboratory test). The observed data confirms Einstein’s general theory of relativity. Credit: Dark Cosmology Centre, Niels Bohr Institute
The next step in the equation is to measure the entire galaxy cluster’s total mass to arrive at its gravitational potential. Then, using the general theory of relativity, the gravitational redshift could be determined by galaxy location.
“It turned out that the theoretical calculations of the gravitational redshift based on the general theory of relativity was in complete agreement with the astronomical observations.” explains Wojtak. “Our analysis of observations of galaxy clusters show that the redshift of the light is proportionally offset in relation to the gravitational influence from the galaxy cluster’s gravity. In that way our observations confirm the theory of relativity.”
Of course, this kind of revelation also has other implications… theoretical dark matter just might play a role in gravitational redshift, too. And don’t forget dark energy. All these hypothetical models need to be taken into account. But, for now, we’re looking at the big picture in a different way.
“Now the general theory of relativity has been tested on a cosmological scale and this confirms that the general theory of relativity works and that means that there is a strong indication for the presence of dark energy”, explains Radek Wojtak.
As Walt Whitman once said, “I open the scuttle at night and see the far-sprinkled systems, And all I see multiplied as high as I can cypher edge but the rim of the farther systems. Wider and wider they spread, expanding, always expanding,Outward and outward and forever outward.”
An image illustrating the number density of galaxies estimated to be four billion light years from the Earth. Bright areas indicate high-density regions. The brightest region in the center corresponds to the main body of the CL0939 cluster. Red squares show the positions of the red -burning galaxies while the greenish-blue dots show the blue H? emitting galaxies. Evidently, the red burning galaxies avoid the central region of the cluster and concentrate in small groups located far away from it.
[/caption]
Utilizing the Subaru Telescope, a research team of astronomers from the University of Tokyo and the National Astronomical Society of Japan (NAOJ) used a wide-field image to take a look four billion years back in time. The object of their interest was a galaxy cluster, but what really took their fancy wasn’t the old matrons – it was the red star-forming galaxies hanging around the edges.
Just exactly what is a “red-burning galaxy”? Astronomers hypothesize they might be the transitional key between the young and old… and present at a party that shows dramatic evolution. It’s not the fact that such galaxies exist within galactic clusters, but why they seem to appear along the outskirts.
When galaxies first began forming under the weight of their own gravity some ten billion years ago, they either became part of big clusters or small groups. As they came together, they took on properties of their environment – just as party goers tend to group together where interests are similar. At a galactic get-together with high density, galaxies form into lenticular or elliptical, while the solitary wall flowers tend toward spiral structure. But exactly how they form and evolve is one of astronomy’s greatest enigmas.
A panoramic view of the CL0939+4713 cluster located 4 billion light years away from Earth. Images were captured with the Subaru Prime Focus Camera (Suprime-Cam), all of which are a composite of a B-band image (blue), a R-band image (green), and a z'-band image (red). Left 27 arcmin x 27 arcmin field of view. Top-right: Close-up view of the central cluster region, 2.5 arcmin x 2.5 arcmin field of view. Bottom-right: Example of the concentration of red-burning galaxies, which are marked with red squares.
To help solve the mystery, researchers are looking further back into the past. A research team led by Dr. Yusei Koyama used the Subaru Prime Focus Camera (Suprime-Cam) to carry out a panoramic observation targeting a relatively well-known rich cluster, CL0939+4713. By using a special filter that separates the hydrogen-alpha emission lline Koyama’s team members identified more than 400 galaxies showing a narrowband excess which could denote the star formation process. Strangely enough, it was these very galaxies that showed an impressive amount of red and were located in groups well away from the main body.
Needless to say, this opened the door to even more questions. Where did they come from and why are they concentrated in groups and not clusters? At this point, who knows? Astronomers are positive the “red-burning galaxies” get their properties from starbirth – not elderly populations. They also anticipate the main galaxy cluster will one day absorb these strays into the main body as well. How can they tell? Just like the party, the red-burning galaxies are already changing in relationship to their environment. Older galaxies that no longer have active star-forming regions seem to be increasing in the groups, exactly where the red-burners are most frequently found.
“This suggests that the red-burning galaxies are related to the increase in old galaxies, and that they are likely to be in a transitional phase from a younger to an older generation. The finding that such transitional galaxies are located most frequently within group environments shows that galaxy groups are the key environments for understanding how environment shapes the evolution of galaxies.” says the Subaru research team. “This should be an important and exciting step toward a more complete understanding of the environments shaping the galaxies in the present-day Universe.”
An infrared/optical representative-color image of a massive galaxy cluster located 7 billion light-years from Earth. Credit: Infrared Image: NASA/JPL-Caltech/M. Brodwin (Harvard-Smithsonian CfA) Optical Image: CTIO Blanco 4-m telescope/J. Mohr (LMU Munich)
[/caption]
Looking back to when our Universe was about half the age it is now, astronomers have discovered the most massive galaxy cluster yet seen at so great a distance. The researchers say that if we could see it as it appears today, it would be one of the most massive galaxy clusters in the universe. The cluster, modestly named SPT-CL J0546-5345, weighs in at around 800 trillion Suns, and holds hundreds of galaxies. “This galaxy cluster wins the heavyweight title,”said Mark Brodwin, from the Harvard-Smithsonian Center for Astrophysics. “This cluster is full of ‘old’ galaxies, meaning that it had to come together very early in the universe’s history – within the first two billion years.”
Using the new South Pole Telescope, Brodwin and his colleagues are searching for giant galaxy clusters using the Sunyaev-Zel’dovich effect – a small distortion of the cosmic microwave background, a pervasive all-sky glow left over from the Big Bang. Such distortions are created as background radiation passes through a large galaxy cluster.
They found the heavyweight cluster in some of their first observations with the new telescope.
Located in the southern constellation Pictor (the Painter), the cluster has a redshift of z=1.07, putting it at a distance of about 7 billion light-years, meaning we see it as it appeared 7 billion years ago, when the universe was half as old as now and our solar system didn’t exist yet.
Even at that young age, the cluster was almost as massive as the nearby Coma cluster. Since then, it should have grown about four times larger.
This optical image of the newfound galaxy cluster highlights how faint and reddened these galaxies are due to their great distance. Credit: CTIO Blanco 4-m telescope/J. Mohr (LMU Munich)
Galaxy clusters like this can be used to study how dark matter and dark energy influenced the growth of cosmic structures. Long ago, the universe was smaller and more compact, so gravity had a greater influence. It was easier for galaxy clusters to grow, especially in areas that already were denser than their surroundings.
“You could say that the rich get richer, and the dense get denser,” quipped Harvard astronomer Robert Kirshner, commenting on the study.
As the universe expanded at an accelerating rate due to dark energy, it grew more diffuse. Dark energy now dominates over the pull of gravity and chokes off the formation of new galaxy clusters.
The main goal of the SPT survey is to find a large sample of massive galaxy clusters in order to measure the equation of state of the dark energy, which characterizes cosmic inflation and the accelerated expansion of the universe. Additional goals include understanding the evolution of hot gas within galaxy clusters, studying the evolution of massive galaxies in clusters, and identifying distant, gravitationally lensed, rapidly star-forming galaxies.
The team expects to find many more giant galaxy clusters lurking in the distance once the South Pole Telescope survey is completed.
Follow-up observations on the cluster were done using the Infrared Array Camera on the Spitzer Space Telescope and the Magellan telescopes in Chile. A paper announcing the discovery has been published in the Astrophysical Journal.
SXDF-XCLJ0218-0510. Max-Planck-Institut für extraterrestrische Physik
[/caption]
Like a location from Star Wars, this galaxy cluster is far, far away and with origins a long, long time ago. With the ungainly name of SXDF-XCLJ0218-0510, this cluster is actually the most distant cluster of galaxies ever seen. It is a whopping 9.6 billion light years away, and X-ray and infrared observations show that the cluster hosts predominantly old, massive galaxies. This means the galaxies formed when the universe was still very young, so finding this cluster and being able to see it is providing new information not only about early galaxy evolution but also about history of the universe as a whole.
An international team of astronomers from the Max Planck Institute for Extraterrestrial Physics, the University of Tokyo and the Kyoto University discovered this cluster using the Subaru telescope along with the XMM-Newton space observatory to look in different wavelengths.
Using the Multi-Object Infrared Camera and Spectrometer (MOIRCS) on the Subaru telescope, the team was able to look in near-infrared wavelengths, where the galaxies are most luminous.
“The MOIRCS instrument has an extremely powerful capability of measuring distances to galaxies. This is what made our challenging observation possible,” said Masayuki Tanaka from the University of Tokyo. “Although we confirmed only several massive galaxies at that distance, there is convincing evidence that the cluster is a real, gravitationally bound cluster.”
Like a contour map, the arrows in the image above indicate galaxies that are likely located at the same distance, clustered around the center of the image. The contours indicate the X-ray emission of the cluster. Galaxies with confirmed distance measurements of 9.6 billion light years are circled. The combination of the X-ray detection and the collection of massive galaxies unequivocally proves a real, gravitationally bound cluster.
That the individual galaxies are indeed held together by gravity is confirmed by observations in a very different wavelength regime: The matter between the galaxies in clusters is heated to extreme temperatures and emits light at much shorter wavelengths than visible to the human eye. The team therefore used the XMM-Newton space observatory to look for this radiation in X-rays.
“Despite the difficulties in collecting X-ray photons with a small effective telescope size similar to the size of a backyard telescope, we detected a clear signature of hot gas in the cluster,” said Alexis Finoguenov from the Max Planck Institute for Extraterrestrial Physics.
The combination of these different observations in what are invisible wavelengths to the human eye led to the pioneering discovery of the galaxy cluster at a distance of 9.6 billion light years – some 400 million light years further into the past than the previously most distant cluster known.
An analysis of the data collected about the individual galaxies shows that the cluster contains already an abundance of evolved, massive galaxies that formed some two billion years earlier. As the dynamical processes for galaxy aging are slow, presence of these galaxies requires the cluster assembly through merger of massive galaxy groups, each nourishing its dominant galaxy. The cluster is therefore an ideal laboratory for studying the evolution of galaxies, when the universe was only about a third of its present age.
As distant galaxy clusters are also important tracers of the large scale structure and primordial density fluctuations in the universe, similar observations in the future will lead to important information for cosmologists. The results obtained so far demonstrate that current near infrared facilities are capable of providing a detailed analysis of distant galaxy populations and that the combination with X-ray data is a powerful new tool. The team therefore is continuing the search for more distant clusters.
A billion years after the big bang, hydrogen atoms were mysteriously torn apart into a soup of ions. Credit: NASA/ESA/A. Felid (STScI)).
It’s no secret that the universe is an extremely vast place. That which we can observe (aka. “the known Universe”) is estimated to span roughly 93 billion light years. That’s a pretty impressive number, especially when you consider its only what we’ve observed so far. And given the sheer volume of that space, one would expect that the amount of matter contained within would be similarly impressive.
But interestingly enough, it is when you look at that matter on the smallest of scales that the numbers become the most mind-boggling. For example, it is believed that between 120 to 300 sextillion (that’s 1.2 x 10²³ to 3.0 x 10²³) stars exist within our observable universe. But looking closer, at the atomic scale, the numbers get even more inconceivable.
At this level, it is estimated that the there are between 1078 to 1082 atoms in the known, observable universe. In layman’s terms, that works out to between ten quadrillion vigintillion and one-hundred thousand quadrillion vigintillion atoms.
And yet, those numbers don’t accurately reflect how much matter the universe may truly house. As stated already, this estimate accounts only for the observable universe which reaches 46 billion light years in any direction, and is based on where the expansion of space has taken the most distant objects observed.
The history of the universe starting the with the Big Bang. Image credit: grandunificationtheory.com
While a German supercomputer recently ran a simulation and estimated that around 500 billion galaxies exist within range of observation, a more conservative estimate places the number at around 300 billion. Since the number of stars in a galaxy can run up to 400 billion, then the total number of stars may very well be around 1.2×1023 – or just over 100 sextillion.
On average, each star can weigh about 1035 grams. Thus, the total mass would be about 1058 grams (that’s 1.0 x 1052 metric tons). Since each gram of matter is known to have about 1024 protons, or about the same number of hydrogen atoms (since one hydrogen atom has only one proton), then the total number of hydrogen atoms would be roughly 1086 – aka. one-hundred thousand quadrillion vigintillion.
Within this observable universe, this matter is spread homogeneously throughout space, at least when averaged over distances longer than 300 million light-years. On smaller scales, however, matter is observed to form into the clumps of hierarchically-organized luminous matter that we are all familiar with.
In short, most atoms are condensed into stars, most stars are condensed into galaxies, most galaxies into clusters, most clusters into superclusters and, finally, into the largest-scale structures like the Great Wall of galaxies (aka. the Sloan Great Wall). On a smaller scale, these clumps are permeated by clouds of dust particles, gas clouds, asteroids, and other small clumps of stellar matter.
Representation of the timeline of the universe over 13.7 billion years, and the expansion in the universe that followed. Credit: NASA/WMAP Science Team.
The observable matter of the Universe is also spread isotropically; meaning that no direction of observation seems different from any other and each region of the sky has roughly the same content. The Universe is also bathed in a wave of highly isotropic microwave radiation that corresponds to a thermal equilibrium of roughly 2.725 kelvin (just above Absolute Zero).
The hypothesis that the large-scale universe is homogeneous and isotropic is known as the cosmological principle. This states that physical laws act uniformly throughout the universe and should, therefore, produce no observable irregularities in the large scale structure. This theory has been backed up by astronomical observations which have helped to chart the evolution of the structure of the universe since it was initially laid down by the Big Bang.
The current consensus amongst scientists is that the vast majority of matter was created in this event, and that the expansion of the Universe since has not added new matter to the equation. Rather, it is believed that what has been taking place for the past 13.7 billion years has simply been an expansion or dispersion of the masses that were initially created. That is, no amount of matter that wasn’t there in the beginning has been added during this expansion.
However, Einstein’s equivalence of mass and energy presents a slight complication to this theory. This is a consequence arising out of Special Relativity, in which the addition of energy to an object increases its mass incrementally. Between all the fusions and fissions, atoms are regularly converted from particles to energies and back again.
Atom density is greater at left (the beginning of the experiment) than 80 milliseconds after the simulated Big Bang. Credit: Chen-Lung Hung
Nevertheless, observed on a large-scale, the overall matter density of the universe remains the same over time. The present density of the observable universe is estimated to be very low – roughly 9.9 × 10-30 grams per cubic centimeter. This mass-energy appears to consist of 68.3% dark energy, 26.8% dark matter and just 4.9% ordinary (luminous) matter. Thus the density of atoms is on the order of a single hydrogen atom for every four cubic meters of volume.
The properties of dark energy and dark matter are largely unknown, and could be uniformly distributed or organized in clumps like normal matter. However, it is believed that dark matter gravitates as ordinary matter does, and thus works to slow the expansion of the Universe. By contrast, dark energy accelerates its expansion.
Once again, this number is just a rough estimate. When used to estimate the total mass of the Universe, it often falls short of what other estimates predict. And in the end, what we see is just a smaller fraction of the whole.
