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Astronomy in Year Trillion

We’ve got to come up with a better name that ‘Milkomeda’ to describe what’s going to eventually happen when the Milky Way and Andromeda merge. Remember that Andromeda is one of the galaxies with a blueshift, showing that it is moving toward us. That the merger will probably happen — in about five billion years — appears inevitable, and it’s fascinating to speculate on the evolution of the elliptical galaxy that should result from all this. In fact, Avi Loeb (Harvard-Smithsonian Center for Astrophysics) and colleague T.J. Cox have run computer simulations showing a faint possibility that our Solar System will be pulled into a ‘tidal tail’ of orphan stars and eventually, before the final merger, wind up in the Andromeda galaxy.

But after a series of close passes, the galaxies will most likely begin to intermingle. Loeb is the one behind the Milkomeda coinage, but I’ve also heard the even worse ‘Milkymeda’ and the at least acceptable ‘Andromeda Way.’ There’s plenty of time to work this out, so I put it to Centauri Dreams readers to ponder a poetic and inspirational name for the ultimate elliptical galaxy. By the time it has fully merged, our Sun will be entering its red giant stage, and our Solar System most likely pushed out to 100,000 light years from the new galactic center. That’s four times the current 25,000 light year distance, a move way out beyond the familiar galactic suburbs.

Image: A near galactic-collision between NGC 2207 (left) and IC 2163 captured by the Hubble Space Telescope. Scientists predict the Milky Way will merge with its neighbor Andromeda in about 5 billion years. Credit: NASA and The Hubble Heritage Team (STScI).

Will the descendants of the human race, however constituted, still be around to study the stars? It’s impossible to know, but it’s clear that any astronomers living in the merged galaxy era will have a far different night sky than ours to work with. And that view will hardly be static. Let’s run the process forward as if we had an H.G. Wells-style time machine (or a Loeb-style computer). 100 billion years from now the Sun and many of the stars we are familiar with will have burned out. Moreover, the accelerated expansion of the universe will have pushed many galaxies out past our cosmic horizon, while many of those that can be seen will only grow dimmer.

Loeb talked about that scenario back in 2001, noting that in this era, 100 billion years from now, an astronomer’s view will be reduced to about a thousand members of the local Virgo Cluster and surrounding areas. When a remote galaxy crosses our ‘horizon,’ the light it emits after that point will not be able to reach us. The galaxy will simply be moving too fast for us to see it. As Loeb says, “This process is analogous to what you see if you watch a light source fall into a black hole. As an object crosses the black hole’s event horizon, its image seems to freeze and fade away because you can’t see the light it emits after that point.”

Trillion Year Spree

Galaxies will slowly disappear, their image frozen and fading. It’s a chilling prospect, but Loeb’s latest paper takes us into an even more remote scenario, fully one trillion years from now, when the universe is 100 times older than it is today. By then the photons of the Cosmic Microwave Background will have a wavelength longer than the visible universe, and all other galaxies will be lost to our view. But Loeb believes that the astronomers of this era will still be able to figure out the Big Bang and the existence of dark matter by studying hypervelocity stars flung out from the galaxy.

Flung out, that is, from the center of the inelegantly named Milkomeda. Here’s the scenario: When a binary star system gets too close to the black hole at galactic center, one star falls into the black hole while the other is thrown outward at speeds high enough to cause it to be ejected from the galaxy. This occurs roughly every 100,000 years, and sharp-eyed future astronomers will be able to use these hypervelocity stars to infer the accelerated expansion of the universe as the stars move beyond the galaxy’s gravitational pull. Advanced technologies measuring that acceleration should make it possible to infer an expanding universe and, in Loeb’s view, calculate the age of the universe and key parameters like the cosmological constant.

“We used to think that observational cosmology wouldn’t be feasible a trillion years from now,” says Loeb. “Now we know this won’t be the case. Hypervelocity stars will allow Milkomeda residents to learn about the cosmic expansion and reconstruct the past… Astronomers of the future won’t have to take the Big Bang on faith. With careful measurements and clever analysis, they can find the subtle evidence outlining the history of the universe.”

Beyond the evidence afforded by hypervelocity stars, other possibilities come to mind, as Loeb outlines in his new paper. Consider all the possible sources of information:

The existence of an early radiation-dominated epoch could be inferred by measuring the abundance of light elements in metal-poor stars and interpreting it with a theory of Big Bang nucleosynthesis. The mass fraction of baryons within Milkomeda could be assumed to be representative of the mean cosmic value at early times. The nucleosynthesis theory can then be used to find the necessary radiation temperature Tγ ∝ a−1 , such that the correct light element abundances would be produced. This would lead to an estimate of the time when matter and radiation had the same energy densities. Since density perturbations grew mainly after that time, it will be possible to estimate the amplitude of the initial density fluctuation on the mass scale of Mtot that was required for making Milkomeda at a time (t − t ) ∼ Hv-1 after the Big Bang. Without a radiation-dominated epoch, this amplitude could have been arbitrarily low at arbitrarily early times. Future astronomers may already have cosmology texts available to them, but even if they do not, we have outlined a methodology by which they will be able to arrive at, and empirically verify, the standard cosmological model.

Loeb sketches out a remote futurity indeed, but it’s a comforting thought that science will still be able to unlock cosmological mysteries even when the compelling view of other galaxies is long gone. You can follow this up in Loeb’s paper “Cosmology with Hypervelocity Stars,” accepted by the Journal of Cosmology and Astroparticle Physics and available as a preprint. And please, for the good of our remote descendants, give some thought to a name more elegant than ‘Milkomeda.’


Comments on this entry are closed.

  • John Gibbons April 14, 2011, 10:37

    How about Via Andromeda? Or if you want to keep closer to consistency (i.e. Greek for both parts, but with English spelling) Andromeda Kyklos or Andromeda Cyclos.

  • Tom Mazanec April 14, 2011, 10:42

    Will “Milkomeda” be a quasar when the central black holes merge? What a view…a quasar withing 100,000 light years of an Earth, with a reddened, obese sun.
    And in 100 billion years, a reddened white dwarf sun, with a ring around the Earth from a tidally disrupted moon?

  • Richard Burke-Ward April 14, 2011, 10:48

    “Milkomeda” is inexcusably bad.

    How about “Perses”? In Greek mythology, that’s the name of Andromeda’s son.

    Love the site, by the way. Big fan.


  • Istvan April 14, 2011, 10:58

    Yet another reason to pay attention to robust methods of data storage and concepts such as the Long Now. Our distant descendants should not be expected to have to deduce the nature of reality from first principles, but rather to have the benefit of 100 billion or more years of scientific progress and retained memory.

    Imagine how far they may see, standing on the shoulders of 100 billion years of giants. It boggles the mind.

  • Erik Anderson April 14, 2011, 12:37

    We live in an era of ignorance. Until we can measure the transverse velocity of the Andromeda Galaxy, its prospective merger with the Milky Way is pure speculation. And portrayals of cosmic scenery one trillion years from now merit no credence at a time when cosmologists are repeatedly blindsided by how the universe is observed to appear just 13.5 billion years ago. Very recent observations now indicate that the first stars in the universe were already shining just 200 million years after the Big Bang. How far back in time the earliest stars will be dated with JWST is anybody’s guess. The entire chronology of the universe may have to be recalibrated, and the notion of “accelerating expansion” may be the first casualty.

  • ljk April 14, 2011, 13:30

    Milkomeda – isn’t that the stuff people take when they are feeling “irregular”?

    Don’t worry, no one will get this one trillion years from now.

  • ljk April 14, 2011, 13:35

    Oh wait, you asked for a better name than Milkomeda.

    How about, in the trail blazed by the Big Bang, we call the galactic merger The Shmush? Because by 1 Trillion CE, won’t a lot of other local galaxies have also merged with the Milky Way and Andromeda? We have to be fair to all of them and I think The Shmush relays the basic elements of the event and is generic enough for the job.

  • Kenneth Mark Hoover April 14, 2011, 13:53

    The term “Milkomedia” is, of course, totally idiotic and downright twee. (Which in itself is akin to a punishable crime, as far as I am concerned.)

    But, typically, this is the type of inane “popular science” garbage I often come to expect now from people who want to ramp up excitement about something, regardless how it might debase the true fundamentals of the science in question.

    Sadly, it’s also another aspect of popularizing astronomy at the expense of what is verifiable, throwing out ignorant and fantastical assumptions without any observational basis or factual evidence gained through repeatable and rigorous experiment . In other words, it sounds good, therefore let’s write an “article” about astronomy that would, in effect, be better served as hoary science fiction from the days of Hugo Gernsback.

    Sadly, this is so typical nowadays I no longer possess the capacity to be surprised when I see it, but my disgust endures.


  • Andrew W April 14, 2011, 16:07

    Government action is urgently required to address this issue.

  • Paul Gilster April 14, 2011, 16:30

    I like the new entries for naming the Milky Way / Andromeda conglomeration, and especially Larry’s ‘The Shmush,’ which seems to state the situation clearly! But Perses sticks with me, especially being the son of Andromeda (and Perseus). Via Andromeda also has a nice classical sound. Hard to choose, but as I say, we have time…

  • Astronist April 14, 2011, 17:02

    I’m happy with “Milkomeda” (pron. mil-KOMM-eda). Would be interested to see the scientific evidence for its being “idiotic” or “inexcusably bad”. Not that it matters; by the time the galaxy forms, nobody will be speaking English any more!

    I agree with Erik Anderson above. When we can’t even explain the rotation of our own and other galaxies except with hypothetical invisible matter, it seems a little premature to pronounce on how the entire universe will look 100 bn or a trillion years in the future.

    Clearly, either we will have no descendants at all at that remote epoch, or the majority of them will have very much better astronomical knowledge than we do. But a branch of our descendants falling into decline and being forgotten by their cousins, so that they have to work out the sciences afresh for themselves, is perhaps conceivable.

    A question: the dwarf galaxies in the Local Group are neither here nor there (unless one comes close to the Solar System), but what about the Triangulum Galaxy? According to Wikipedia, it’s approaching us and also approaching Andromeda. So do all three spirals eventually merge into one? Which merger happens first?

    Oxford, UK

  • Astronist April 14, 2011, 17:03

    Oh, a name. I would call it the Milkomedulum Galaxy…


  • Tobias Holbrook April 14, 2011, 18:11

    Well, the Milky Way isn’t a very classical name… my vote goes to The Shmush. Or maybe name it after some other chocalate?

  • Rob Henry April 14, 2011, 18:40

    As Erik Anderson, put it what is Andromeda’s transverse motion, or as I was thinking, is it even possible to measure the proper motion of an object so distant? Is this merger hypothesis really an “either Andromeda and the Milky Way are not both gravitationally bound to the Local Group or they will merge” hypothesis, or is there some deeper calculation?

  • Bounty April 14, 2011, 20:02

    I’m guessing by then we will have either found ET or have 100% sole responsibility for the name of galaxies. In other words, by the time they merge, we may be calling our galazy Shmushineshyhse 9 (that’s alien for spiral #9.) If we find ourselves alone, I say stick with Milky Way, since it will eventually look like a milky way once the merger is complete.

  • Mike Miller April 14, 2011, 20:05


    How’s about the “Mike Miller Galaxy”?


  • Chris Willrich April 14, 2011, 22:01

    Pandromeda? By analogy with Pangaea, and emphasizing that Andromeda is the larger galaxy. Echoing Richard Burke-Wood, I love Centauri Dreams. I just rarely have anything to contribute beyond silly names.

  • Peter Popov April 14, 2011, 23:56

    A similar piece by Loeb appeared in the online version of newscientist some time ago. I was shocked by the wild claims and explicit details of how the two galaxies will merge, down to predicting where a particular star (the Sun) will end up. The article which discusses “Milkomeda” is actually this one”T. J. Cox, & A. Loeb, Mon. Not. R. Astron. Soc. 386, 461, 2008″, it is reference [4] in the preprint above. Here is why most of the claims regarding Milkomeda are outrageous:

    1. Nobody knows what is the actual relative velocity between the centers of mass of the two galaxies. This is acknowledged by the authors.
    2. Nobody knows or can conceivably find out, with sufficient precision, what is the pointwise mass density distribution of either galaxy and the intergalactic medium.

    Any prediction, regardless of the numeral approximation employed, will be extremely sensitive to those two parameters. Therefore, any claim concerning the eventual position of individual stars is simply ridiculous. It is also ridiculous to take all stars at a certain distance from the center of the Milky Way, as the authors do, and then count the percentage that end up in a particular location. Hence a probability is concluded as to what will happen to our Sun. This is also outrageous. Any meaningful statistical analysis should vary the initial conditions. Since the phase space of possible initial conditions is (exponentially) huge, direct Monte Carlo style parametric studies are beyond any computer that will ever be build.

    In addition to the above, this paper also simulates numerically unknown physics, namely dark matter. Now, nobody knows what dark matter is, what its properties are, etc. All there is about it are indirect indications of missing matter density, from which the presence of dark matter is inferred. Now, according to Loeb, the main reason why the two galaxies will collide in the first place is as follows: “… This outcome [the merger] appears inevitable given the massive halos of dark matter that likely surround the Milky Way and Andromeda …”. All that this paper does is assume some arbitrary properties of some substance (dark matter), which may or may not exists, and if it exists its properties are yet to be characterized in a lab. Even if all of the preceding items are guessed correctly, Loeb then has to assumes an initial density distribution of this mysterious stuff around the two galaxies. Based on all of the above, it is claimed they will collide. This may or may note be so. In fact, dark matter is not the issue when it comes to accuracy of such forward simulations as done in the paper: the density of observable matter in both galaxies is either inaccurate (e.g. no exotic matter) or some exotic stuff exists (e.g. dark matter) to compensate for observations, or some physics is missing/wrong. Clearly, until this is resolved, making claims on forward simulations, e.g. simulating the long-term dynamics of two galaxies, is pure speculation. While such may have some limited value regarding the possible properties of the unknown stuff, making claims on individual star positions is ludicrous.

  • Stevo Darkly April 15, 2011, 0:24


  • Spill April 15, 2011, 2:32

    I say the worse name the better, so my preference is currently Milkymeda.

    That way its another incentive to make sure we get colonising, with the focus on getting beyond galaxy. After all who would want to have an address in the Milkymeda galaxy when there are so many other better named galaxies to chose from.

  • ljk April 15, 2011, 11:27

    The Calvin and Hobbes suggestion for the Big Bang could also apply here: The Horrendous Space Kablooie!

    If the galaxies do not merge naturally, really advanced ETI in the far future may merge them purposely in order to consolidate resources and such, since most other galaxies will be moving far out of sight.

    The Galactic Dyson Shell has also been considered, but since a mere Dyson Shell wigs some humans out, I will withold further discussion on the subject for now.

  • Alexander April 15, 2011, 12:36

    How about Milk Shake Galaxy?

  • Roger April 16, 2011, 9:17

    If worm holes could exist and went to galaxies that were later pushed out past the cosmic horizon, wouldn’t you still have a way to get there? Maybe some ancient alien civilisation has already done this and it is possible for them to reach all parts of the Universe? There would be a large motivation to connect the galaxies before they are lost past the shrinking cosmic horizon.

  • bigdan201 April 16, 2011, 16:50

    I feel that we still need to iron out more information on dark matter/dark energy, and how they relate. Until then, I would argue that we can’t be 100% certain of long-term projections of the universe, even if they have strong backing.

    Learning more about the motions of galaxies will be useful, particularly if our descendants ever try to jump across the intergalactic void.

    As for the potential merged galaxy… Milky Way comes from the latin Via Lactea, so I suggest Androlactea.

  • andy April 16, 2011, 17:04

    Apparently the transverse motion of M33 is known thanks to measurements of masers in the galaxy. From what I can tell from a brief search, insufficient numbers of masers are known in M31 to do a similar measurement.

  • Mike Miller April 17, 2011, 3:25


    …. or, we could call it ” The Milkier Way “.


  • Jonathan Burns April 18, 2011, 8:24

    “The Galaxy Waltz” has a pleasant cadence, I feel.

  • Malcolm Ramsay April 19, 2011, 10:59

    Since they’re both of Greek origin (“Milky Way” deriving from “Galaxias”), how about Andromaxias?

  • Malcolm Ramsay April 19, 2011, 10:59

    Or “Galomeda”?

  • Ronald April 20, 2011, 12:30

    While actual collisions of stars will be extremely rare due to the enormous distances between the individual stars, the future merger of the MW with Andromeda will still have at least one enormous impact on its future composition: it is known that the elliptical galaxies resulting from a merger of two (or more) galaxies go through a relatively very short period of very accellerated star formation, a true burst. It is, as it were, as if such an elliptical galaxy uses up most of its remaining hydrogen reserves for star formation very quickly, followed then by very low star formation for the remainder of its existence.
    This in turn would have profound consequences for the possibilities of future life and intelligence in such a galaxy: the sunlike stars would be gone relatively quickly and eventually the galaxy would consist almost exclusively of long-lived red and orange-red dwarfs.

    Furthermore, I think that in about 100 gy most if not all of the galaxies in our supercluster, the Virgo supercluster, will have merged into one supergalaxy, since, if I am not mistaken, all galaxies in one supercluster are gravitationally bound and the universe is expanding only at the level beyond the supercluster (i.e. the different superclusters are moving apart).

    There was an interesting article in Scientific American about this topic in March 200b, by Krauss & Scherrer (also mentioning Loeb), called “The End of Cosmology?”.

    Anyway, it looks as if the 7x present age of the universe and beyond will consist mainly of long-lived red and orange-red dwarfs (roughly K5 and later), so I hope that earthlike planets and life are possible around such stars.
    Unless a very advanced civilization would be able to merge individual stars to bigger ones, in which case such red dwarfs would constitute a magnificent long-term and low-waste ‘cosmic reserve’.

  • hiro April 20, 2011, 16:20

    We don’t even know what the Great Attractor really is; predicting the dynamic of the local universe in 1trillion years from now in detail is just hopeless. By the way, we aren’t the only player in town, I believe there are some advanced civilizations in Virgo and nearby superclusters (Coma, Hydra-Centaurus, Perseus-Pisces, Shapley etc…) and human species is either extinct or becoming software mind.

  • ljk April 20, 2011, 17:21

    Ronald, to go much further than what you describe above, if the Universe just keeps spreading apart and there is no Big Crunch or Brane Smack, one day there will only be massive (and I mean massive) black holes left. Even they will one day evaporate, leaving the Universe with a few random atoms and protons bouncing around in a void that makes our current intergalactic voids look like tight quarters by comparison.

    Now it has been shown that advanced civilizations can use black holes both for energy and as a place to dump their waste. In fact the process of getting rid of their excess into the black hole is what will power their societies circling the celestial monstrosities at a safe distance, of course. So any beings left in that utterly distant epoch may at least last as long enough as the black holes do. After that they better hope there are other, younger universes they can get to or make their own baby universe and hop into it. I will also presume that most advanced ETI which could escape the end of our Universe will probably do so long before things get really, literally dark.

    Winding things back to just a mere few billion years from now, I hope our Milky Way – Andromeda Galaxy merger – whatever it may be called – will look as pretty as the one captured and just released by the HST team here:


  • Mike April 21, 2011, 15:19

    Clearly, it’ll become the Butter Galaxy because the Milky Way will get churned.

  • ljk October 12, 2012, 13:30

    11 October 2012

    ** Contacts are listed below. **

    Text & Images:



    Using gravitational “lenses” in space, University of Utah astronomers discovered that the centers of the biggest galaxies are growing denser — evidence of repeated collisions and mergers by massive galaxies with 100 billion stars.

    “We found that during the last 6 billion years, the matter that makes up massive elliptical galaxies is getting more concentrated toward the centers of those galaxies. This is evidence that big galaxies are crashing into other big galaxies to make even bigger galaxies,” says astronomer Adam Bolton, principal author of the new study.

    “Most recent studies have indicated that these massive galaxies primarily grow by eating lots of smaller galaxies,” he adds. “We’re suggesting that major collisions between massive galaxies are just as important as those many small snacks.”

    The new study — published recently in The Astrophysical Journal http://dx.doi.org/10.1088/0004-637X/757/1/82 — was conducted by Bolton’s team from the Sloan Digital Sky Survey-III using the survey’s 2.5-meter optical telescope at Apache Point, N.M., and the Earth-orbiting Hubble Space Telescope.

    The telescopes were used to observe and analyze 79 “gravitational lenses,” which are galaxies between Earth and more distant galaxies. A lens galaxy’s gravity bends light from a more distant galaxy, creating a ring or partial ring of light around the lens galaxy.

    The size of the ring was used to determine the mass of each lens galaxy, and the speed of stars was used to calculate the concentration of mass in each lens galaxy.

    Bolton conducted the study with three other University of Utah astronomers — postdoctoral researcher Joel Brownstein, graduate student Yiping Shu and undergraduate Ryan Arneson — and with these members of the Sloan Digital Sky Survey: Christopher Kochanek, Ohio State University; David Schlegel, Lawrence Berkeley National Laboratory; Daniel Eisenstein, Harvard-Smithsonian Center for Astrophysics; David Wake, Yale University; Natalia Connolly, Hamilton College, Clinton, N.Y.; Claudia Maraston, University of Portsmouth, U.K.; and Benjamin Weaver, New York University.

    Big Meals and Snacks for Massive Elliptical Galaxies

    The new study deals with the biggest, most massive kind of galaxies, known as massive elliptical galaxies, which each contain about 100 billion stars. Counting unseen “dark matter,” they contain the mass of 1 trillion stars like our Sun.

    “They are the end products of all the collisions and mergers of previous generations of galaxies, perhaps hundreds of collisions,” Bolton says.

    Despite recent evidence from other studies that massive elliptical galaxies grow by eating much smaller galaxies, Bolton’s previous computer simulations showed that collisions between large galaxies are the only galaxy mergers that lead, over time, to increased mass density on the center of massive elliptical galaxies.

    When a small galaxy merges with a larger one, the pattern is different. The smaller galaxy is ripped apart by gravity from the larger galaxy. Stars from the smaller galaxy remain near the outskirts — not the center — of the larger galaxy.

    “But if you have two roughly comparable galaxies and they are on a collision course, each one penetrates more toward the center of the other, so more mass ends up in the center,” Bolton says.

    Other recent studies indicate stars are spread more widely within galaxies over time, supporting the idea that massive galaxies snack on much smaller ones.

    “We’re finding galaxies are getting more concentrated in their mass over time even though they are getting less concentrated in the light they emit,” Bolton says.

    He believes large galaxy collisions explain the growing mass concentration, while galaxies gobbling smaller galaxies explain more starlight away from galactic centers.

    “Both processes are important to explain the overall picture,” Bolton says. “The way the starlight evolves cannot be explained by the big collisions, so we really need both kinds of collisions, major and minor — a few big ones and a lot of small ones.”

    The new study also suggests the collisions between large galaxies are “dry collisions” — meaning the colliding galaxies lack large amounts of gas because most of the gas already has congealed to form stars — and that the colliding galaxies hit each other “off axis” or with what Bolton calls “glancing blows” rather than head-on.

    Sloan Meets Hubble: How the Study Was Conducted

    The University of Utah joined the third phase of the Sloan Digital Sky Survey, known as SDSS-III, in 2008. It involves about 20 research institutions around the world. The project, which continues until 2014, is a major international effort to map the heavens as a way to search for giant planets in other solar systems, study the origin of galaxies and expansion of the universe, and probe the mysterious dark matter and dark energy that make up most of the universe.

    Bolton says his new study was “almost gravy” that accompanied an SDSS-III project named BOSS, for Baryon Oscillation Spectrographic Survey. BOSS is measuring the history of the universe’s expansion with unprecedented precision. That allows scientists to study the dark energy that accelerates expansion of the universe. The universe is believed to be made of only 4 percent regular matter, 24 percent unseen “dark matter” and 72 percent yet-unexplained dark energy.

    During BOSS’ study of galaxies, computer analysis of light spectra emitted by galaxies revealed dozens of gravitational lenses, which were discovered because the signatures of two different galaxies are lined up.

    Bolton’s new study involved 79 gravitational lenses observed by two surveys:

    — The Sloan Survey and the Hubble Space Telescope collected images and emitted-light color spectra from relatively nearby, older galaxies — including 57 gravitational lenses — 1 billion to 3 billion years back into the cosmic past.

    — Another survey identified 22 lenses among more distant, younger galaxies from 4 billion to 6 billion years in the past.

    The rings of light around gravitational-lens galaxies are named “Einstein rings” because Albert Einstein predicted the effect, although he wasn’t the first to do so.

    “The more distant galaxy sends out diverging light rays, but those that pass near the closer galaxy get bent into converging light rays that appear to us as of a ring of light around the closer galaxy,” says Bolton.

    The greater the amount of matter in a lens galaxy, the bigger the ring. That seems counterintuitive, but the larger mass pulls with enough gravity to make the distant star’s light bend so much that lines of light cross as seen by the observer, creating a bigger ring.

    If there is more matter concentrated near the center of a galaxy, the faster stars will be seen moving toward or being slung away from the galactic center, Bolton says.

    Alternative Theories

    Bolton and colleagues acknowledge their observations might be explained by theories other than the idea that galaxies are getting denser in their centers over time:

    — Gas that is collapsing to form stars can increase the concentration of mass in a galaxy. Bolton argues the stars in these galaxies are too old for that explanation to work.

    — Gravity from the largest massive galaxies strips neighboring “satellite” galaxies of their outskirts, leaving more mass concentrated in the centers of the satellite galaxies. Bolton contends that process is not likely to produce the concentration of mass observed in the new study and explain how the extent of that central mass increases over time.

    — The researchers merely detected the boundary in each galaxy between the star-dominated inner regions and the outer regions, which are dominated by unseen dark matter. Under this hypothesis, the appearance of growing galaxy mass concentration over time is due to a coincidence in researchers’ measurement method, namely that they are measuring younger galaxies farther from their centers and measuring older galaxies closer to their centers, giving an illusion of growing mass concentration in galactic centers over time. Bolton says this measurement difference is too minor to explain the observed pattern of matter density within the lens galaxies.

    PIO Contact:
    Lee Siegel
    Science News Specialist
    University of Utah Communications
    +1 (801) 581-8993, cell: +1 (801) 244-5399

    Science Contact:
    Adam Bolton
    Assistant Professor of Physics and Astronomy
    +1 (801) 585-5383, cell +1 (808) 343-0404