Messier Monday: The Biggest One of them All, M87

Of all the galaxies in our local supercluster, one outweighs them all.


“I recognize my limits, but when I look around I realise I am not living, exactly, in a world of giants.” -Giulio Andreotti

Each Monday, we’ve chosen one of the night sky wonders that makes up the Messier catalogue, the first accurate catalogue of deep-sky objects easily visible with a small, amateur telescope from Earth. But of all these objects — from star clusters to planetary nebulae, from globular clusters to supernova remnants, and from star-forming regions to entire galaxies — only one can be the largest. Today, that’s exactly what we’re going to shoot for.

Image credit: Tenho Tuomi of Tuomi Observatory, via http://www.lex.sk.ca/astro/messier/index.html.

The vast majority of the deep-sky objects visible from Earth are contained either within the disk or the halo of our own galaxy, and with good reason: closer things appear brighter to us. We’re located on the outer edge of a tendril of our local supercluster, with only a few dozen modest galaxies within tens of millions of light years. But some 50-to-60 million light years distant lies the Virgo Cluster, the most massive and dense cluster of galaxies in our local Universe. And at the very heart of it lies the biggest, most massive galaxy within hundreds of millions of light years of our home: Messier 87.

As dedicated skywatchers try and catch all 110 Messier objects (and you can participate remotely), make sure you don’t miss the biggest one. Here’s how to find it.

Image credit: me, using the free software Stellarium, available at http://stellarium.org/.

Following the arc of the Big Dipper’s handle, you’ll arrive at the orange giant star Arcturus, the brightest star in the Northern Hemisphere, and then can continue on to bright blue Spica, which is situated right next to Mars at the present time. But below the “bowl” of the Big Dipper lies the prominent constellation of Leo the Lion, heralded by its brightest members Regulus and Denebola. If you draw an imaginary line from Regulus through Denebola and head towards the space between Arcturus and Spica, you’ll come to the slightly less bright (but still prominent) star Vindemiatrix, and it’s between Vindemiatrix and Denebola you’ll need to look for Messier 87.

Image credit: me, using the free software Stellarium, available at http://stellarium.org/.

Most of the stars along that path are invisible to the naked eye, but under relatively clear-and-dark skies, 6 Comae Berenices and ρ Virginis — labelled above — can clearly be spotted. If you draw an imaginary line connecting Denebola to Vindemiatrix and another one connecting 6 Comae Berenices to ρ Virginis, there will be one point where they intersect. And that’s where you should point your telescope.

Image credit: me, using the free software Stellarium, available at http://stellarium.org/.

Although there aren’t any bright stars of note in this region, there are a slew of dimmer stars, but also a plethora of faint, fuzzy, extended nebulous regions: the galaxies of the Virgo Cluster. Through long-exposure astrophotography, this region simply sparkles with wonder.

Image credit: Rogelio Bernal Andreo of Deep Sky Colors, via http://www.deepskycolors.com/archive/2009/01/31/virgo-Galaxy-Cluster.html.

And at the heart of the cluster lies its single, most massive galaxy of all: the giant elliptical galaxy Messier 87. Although it didn’t appear so spectacular to Messier himself — when he discovered it in 1781 — we sure do know an awful lot more about it now. That, and we have some images and data that really show us how impressive this behemoth truly is!

Image credit: Rogelio Bernal Andreo of Deep Sky Colors again, via APOD at http://apod.nasa.gov/apod/ap110422.html.

For starters, I want you to think of our Milky Way: a spiral galaxy that’s around 100,000 light-years across, with hundreds of billions of stars, maybe a trillion Suns’ worth of mass, and a couple of hundred globular clusters in its halo. Oh, and a supermassive black hole at the center that, on its own, contains the mass of four million Suns. In the future, our entire local group will merge together, maybe tripling the Milky Way’s current mass, adding hundreds more globular clusters, hundreds of billions more stars and growing our black hole into the tens of millions of solar masses. That’s our far future, when everything that’s gravitationally bound to us merges.

Image credit: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas and A. Mellinger.

Now, let’s compare that to Messier 87 as it is right now.

Image credit: © 1999-2009 — RC Optical Systems, Robert Gendler, via http://gallery.rcopticalsystems.com/gallery/m87.html.

Messier 87 is a giant elliptical galaxy, the type most commonly found after a series of major mergers of large, Milky Way-like spirals. In the center of the Virgo Cluster, elliptical galaxies dominate, and can reach proportions far larger than their spiral predecessors ever did. How big is “giant,” you ask?

Image credit: © 2006 — 2012 by Siegfried Kohlert, via http://www.astroimages.de/en/gallery/M87.html.

Try half-a-million light-years in diameter, or about five times the extent, in all directions, the longest direction of the Milky Way. Its mass? A whopping two hundred times that of our galaxy today, and about 70 times as great as our entire local group. And as far as globular clusters go? Take a look at this image, below.

Image credit: NASA / Hubble, Wikisky snapshot tool, via Wikimedia Commons user Friendlystar.

Do you see those little “points” distributed through the halo of this behemoth, that look like they could be bright stars? Those aren’t stars at all, but dense collections of hundreds of thousands of stars each, i.e., globular clusters, and there are around 12,000 of them in Messier 87 alone. And finally, did you notice something unique about the center of this galaxy?

Image credit: Brad Bates of Brook Mar Observatory, via http://www.brookmarobservatory.com/M87J_2010.html.

There’s a jet of ionized plasma streaming from the center, spanning 5,000 light-years on its own! What could cause this? Why, a supermassive black hole that dwarfs our own, of course! This is something that becomes apparent if we look in many different wavelengths, as both the X-ray and radio show the telltale signs of an active black hole.

It’s the largest one in our nearby Universe, with an estimated mass of four billion Solar masses, or about a thousand times as massive as our own galaxy’s! The radiation across many different wavelengths coming from its center has been one of the most educational things for astronomy as far as teaching us that these jets, active galaxies and quasars are likely to be different manifestations, magnitude and orientations of the exact same phenomenon: matter being accelerated by a supermassive black hole.

Image credit: NASA and The Hubble Heritage Team (STScI/AURA).

But there’s one way that the Milky Way has this galaxy beat: the amount of dust.

Interstellar dust is made up of neutral molecules capable of blocking light, and our Milky Way has hundreds of millions of Solar masses worth in it. But the intense X-rays emitted by the core of Messier 87 destroys any new dust on timescales of just tens of millions of years; at most, there are only 70,000 Solar masses worth of dusty material in this galaxy at present.

In the future, as the various galaxies in this cluster merge together, adding on to Messier 87, it will most likely grow to more than ten times its current mass, topping the quadrillion-solar-mass mark in time. It’s the largest galaxy for hundreds of millions of light years, for sure. But it’s got a long way to go before it challenges the record-holder for biggest galaxy in the known Universe!

Image credit: X-ray: NASA/CXC/UCI/A.Lewis et al. Optical: Pal.Obs. DSS; via http://chandra.harvard.edu/photo/2003/abell2029/.

That honor goes to IC 1101, which is nearly three times the extent of Messier 87 in its longest direction, and contains about four times as many stars. But IC 1101 is some twenty times farther away than Messier 87, at over a billion light-years distant.

So when you start to feel like the world is a small place, remember that there are bigger things out there than even our galaxy, and even than the biggest galaxies in our neighborhood. The Universe is full of surprises, wonders, and magnificent stories that it tells us about itself, if only we remember to ask the right questions and listen to its answers.

Image credit: John C. Smith of Hidden Loft, via http://www.hiddenloft.com/M/M87.htm.

And that’s the cosmic story of our supercluster’s greatest galaxy! Have a look back at all our previous Messier Mondays:

And come back next time for yet another; we’re going to get all 110 by the end of the year!


Have a comment? Drop in to the Starts With A Bang forum on Scienceblogs!

Next Story — Science: where finding nothing is the biggest victory of all
Currently Reading - Science: where finding nothing is the biggest victory of all

A Higgs boson decaying into Fermions — taus and bottom quarks — in 2012 at the CMS detector. The Higgs boson is the last new particle discovery, and it completes the Standard Model. Image credit: CERN / CMS collaboration.

Science: where finding nothing is the biggest victory of all

“Eureka!” is not always as powerful as “that’s what I thought!”


“Reality is what kicks back when you kick it. This is just what physicists do with their particle accelerators. We kick reality and feel it kick back. From the intensity and duration of thousands of those kicks over many years, we have formed a coherent theory of matter and forces, called the standard model, that currently agrees with all observations.” -Victor J. Stenger

Reading the science news over the past month, you might conclude that it’s been one defeat after another for physics. After all:

Yet these apparent defeats are merely a thin veil covering the greatest truth of all: physics really is incredibly well-understood.

Image credit: E. Siegel, of the known particles in the Standard Model. This is still everything that’s been directly discovered.

From a particle physics perspective, the Standard Model describes all the normal matter we’ve ever observed or detected directly. Combined with General Relativity, our leading theory of gravitation, the four fundamental forces describing all the particles and their interactions — the strong nuclear, electromagnetic, weak nuclear and gravitational forces — are almost completely understood. To be honest, they’re so well understood that most people take this for granted.

The fabric of spacetime, illustrated, with ripples and deformations due to mass. Image credit: European Gravitational Observatory, Lionel BRET/EUROLIOS.

General Relativity was formulated in the 1910s; the Standard Model’s predictions were finalized in the 1960s. For the past 50 years, the greatest novel ideas in theoretical physics, from grand unification to neutrinoless double beta decay to extra dimensions to supersymmetry, have all failed to turn up a direct experimental signature of a new particle or interaction beyond the known forces. We’ve reached the point where the only the most esoteric questions respecting the matter we know, questions concerning the gravitational field of an electron passing through a double slit or the information from the particles falling into a black hole, for example, aren’t answered by our current theories.

We don’t know what happens at the singularity inside a black hole, either, but we’re a long way from gathering experimental data about that! Image credit: NASA, via http://www.nasa.gov/topics/universe/features/smallest_blackhole.html.

If you put any Standard Model particle or set of particles out into the Universe and subject it to any sort of conditions at any energies, if you allow it to collide in a controlled or uncontrolled fashion with low energy, high energy or ultra-high energy particles, you can describe every single interaction by these simple sets of laws. If you shield these particles from everything you can conceive of in the Universe, you see exactly what these laws predict.

The key thing — the greatest victory — is that we don’t see anything elseat all.

The observed Higgs decay channels vs. the Standard Model agreement, with the latest data from ATLAS and CMS included. The agreement is astounding. Images credit: André David, via Twitter at https://twitter.com/DrAndreDavid/status/747858989367595009.

We don’t see any unexpected or unanticipated decays. We don’t see any collisions whose properties we can’t explain. We don’t see the violation of laws or symmetries we don’t expect. We don’t see even tiny amounts of a signal that’s forbidden by the Standard Model or General Relativity. We don’t see proton decay; we don’t see flavor-changing-neutral-currents; we don’t see CPT violation; we don’t see anything move faster than the speed of light in a vacuum. And when it comes to the new things that we do see, they’re exactly in line with what’s predicted, from gravitational frame-dragging to the Higgs boson decaying exactly as we expect to pulsars spinning in perfect time to gravitational radiation matching up perfectly with what Einstein predicted 101 years prior.

The inspiral and merger gravitational wave signal extracted from the event on December 26, 2015. Image credit: Figure 1 from B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), Phys. Rev. Lett. 116, 241103 — Published 15 June 2016.

There are lots of stories out there about how physics is broken, and how we need a massive new breakthrough or a paradigm shift to keep the enterprise of new discoveries going. What nonsense! The truth is that the laws of physics we have in place are the most successful sets of laws we’ve ever come up with. They’ve been tested more robustly than any other set of laws ever, and they’ve passed every single one. We might not understand why the laws are the way they are, or why the Universe comes with certain properties that the Standard Model and General Relativity don’t have an explanation for, such as:

  • dark matter,
  • dark energy,
  • the tiny, non-zero masses of neutrinos,
  • or the matter/antimatter asymmetry of the Universe.

There’s more to learn, for certain. There are more questions out there to be answered. What these null results — these non-discoveries — are telling us is something phenomenal and profound: that physics isn’t over and done, but rather that the hints of what comes next requires looking far, far deeper than we’re presently looking. That means higher energies, larger telescopes, more particle collisions, more sensitive detectors, more significant digits closer to the speed of light or absolute zero and quite likely better, newer ideas than the ones we’ve been pursuing fruitlessly for so long.

The farthest galaxy ever spectroscopically confirmed. To push the frontiers even farther, we’ll need to go even deeper into the Universe. Image credits: NASA, ESA, and A. Feild (STScI).

It’s a great opportunity for creative scientists and a wonderful time to be alive. We can honestly look back and marvel at how far we’ve come, what we’ve already discovered, and how miraculously well it all works. At the same time, we can look ahead at the great mysteries in front of us, and contemplate just what marvelous secrets about nature they might hold when we finally unlock them. In the meantime, it’s no nightmare that these answers remain hidden; it’s merely the latest, greatest challenge that the Universe has presented us with. The fact that we’ve found nothing new that’s compelling so far means our journey must continue. The promised land is still ahead.


This post first appeared at Forbes, and is brought to you ad-free by our Patreon supporters. Comment on our forum, & buy our first book: Beyond The Galaxy!

Next Story — How did the Universe get its first supermassive black holes?
Currently Reading - How did the Universe get its first supermassive black holes?

Illustration of the distant galaxy CR7, which last year was discovered to house a pristine population of stars formed from the material direct from the Big Bang. Image credit: M. Kornmesser / ESO.

How did the Universe get its first supermassive black holes?

How they get so big so fast is a mystery. Could “direct collapse” be the solution we’re seeking?


“For something to collapse, not all systems have to shut down. In most cases, just one system is enough.” -Robert Kiyosaki

At the center of almost every large galaxy lies a supermassive black hole, millions or even billions of times the mass of our Sun in scale. Our Universe has been around for 13.8 billion years, which you might think is plenty of time to form a black hole that large. Yet the farther and farther back in time we look, every supermassive black hole we measure seems to have roughly the same mass as the ones today. In other words, while the largest-scale structures in the Universe:

  • giant elliptical galaxies,
  • ultra-massive clusters with thousands of times the mass of the Milky Way,
  • and galactic filaments that are hundreds of millions of light years across,

took billions of years to form, there are ultra-massive black holes that have been around as far back as we can see.

An illustrated timeline of the Universe’s history. Image credit: European Southern Observatory (ESO).

They don’t predate stars, but they are found in the earliest massive galaxies we know how to measure. The crazy explanation is that the Universe was born with these cosmic behemoths, but that flies in the face of everything else we know about structure formation, including the magnitudes and mass/size scales of the fluctuations that gave rise to everything we see today. The new physics that would have to be hypothesized to create a Universe that was born with extremely large black holes is not only preposterous, it’s incredibly constrained by observations of the cosmic background light populating the cosmos.

The fluctuations in the Cosmic Microwave Background, as seen by Planck. Image credit: ESA and the Planck collaboration.

But there are two “mundane” explanations, or explanations that don’t involve any new fundamental physics beyond that which we currently know.

  1. A huge set of starbursts — a giant rush of catastrophic star-formation — triggered the formation of a great many stellar-mass black holes in just a few million years. Over time, they merged together and migrated towards the center of the galaxy, giving rise to a supermassive black hole in very short order.
  2. A supermassive black hole formed by direct collapse of matter into a very large, massive black hole all at once, providing the seed for ultramassive black holes to grow over a relatively short timescale.

The first scenario, by a great many people, is thought to be the simplest explanation, since we see plenty of evidence of how this could work, even today.

The starburst galaxy Henize 2–10, located 30 million light years away. Image credit: X-ray (NASA/CXC/Virginia/A.Reines et al); Radio (NRAO/AUI/NSF); Optical (NASA/STScI).

Star formation is known to occur in bursts, with the greatest rates of star formation occurring in the first three billion years of the Universe, and falling ever since. When stars form en masse, they produce stars of all different masses and colors, including copious numbers of stars over 20, 50, 100 or even 200 solar masses. These massive, blue, hot stars are both the brightest and the shortest lived, and they end their lives in core-collapse supernovae, almost all of which give rise to black holes. Because of the dynamics of gravity, the way these most massive objects work is that they interact with the other stars around them, kick them around while sinking to the center of a cluster-or-galaxy, and then merge together. It’s a simple, conservative scenario. But it might also be insufficient.

Last year, the galaxy CR7 was discovered: a strong candidate for having a truly pristine population of stars. Pristine means that this would be the very first time stars were forming inside this galaxy since the Big Bang, and the science supports this view quite strongly. Yet if we look at even this galaxy, we find something spectacular about it: it also exhibits evidence for a supermassive black hole. And while the starburst explanation is tempting, it may not line up completely with what we see. In a paper written earlier this year, scientists Aaron Smith, Volker Bromm and Abraham Loeb came up with a different explanation: perhaps they were seeing the first evidence for a direct-collapse black hole!

The X-ray and optical images of a small galaxy containing a black hole many tens of thousands of times the mass of our Sun. These black holes may have arisen first in the Universe by the direct collapse of matter. Image credit: X-ray: NASA/CXC/Univ of Michigan/V.F.Baldassare, et al; Optical: SDSS; Illustration: NASA/CXC/M.Weiss.

This galaxy, sending light from 13 billion years ago, has to see that light travel throughout the expanding Universe, where its wavelengths get stretched from ultraviolet through the visible portion of the spectrum and all the way into the infrared. Still, the hottest, bluest stars — which it’s rich in — cause intense ultraviolet emission from the atoms present: hydrogen and helium. These emission lines originate from slightly different parts of the galaxy, and thanks to the incredible technology of the 2015 observations detecting them, we were able to determine that the hydrogen emissions appear to be moving quickly, at 160 km/s, relative to the helium emissions. When Smith, Bromm and Loeb try to model these emissions using simulations, they find that a massive source of radiation must be present at the center, creating an ionized bubble and driving an expanding shell of gas out from the center.

A rich nebula of gas, pushed out into the interstellar medium by the hot, new stars formed in the central region. Image credit: Gemini Observatory / AURA.

There are two explanations to consider: either there’s a massive star cluster at an incredibly high temperature of 100,000 K, or there’s a massive black hole driving it. The big difference between the two models is that the massive black hole produces the offset speed between the hydrogen and helium as well as the incredible size of the region (over 50,000 light years!) while the primordial massive star cluster does not.

These are simulations only, however; if you want to confirm your picture, you need evidence to decide one way or the other. The way we’ll be able to determine whether there truly is a massive black hole will be observational, and it will involve looking for the characteristic radio emissions from black holes.

A small section of the Karl Jansky Very Large Array, one of the world’s largest and most powerful arrays of radio telescopes. Image credit: John Fowler, under a cc-by-2.0 license.

The largest, most advanced radio telescope arrays in the world are on the verge of being up to the task! The evidence pointing to the existence of direct-collapse black holes is tantalizing and suggestive, but we’re not over the threshold yet. In order to get there, we need to see the proof. But the theoretical possibility has been raised, and the gauntlet has been thrown down. It’s time to collect the evidence and let nature decide!


This post first appeared at Forbes, and is brought to you ad-free by our Patreon supporters. Comment on our forum, & buy our first book: Beyond The Galaxy!

Next Story — Cassini prepares for its final, suicidal mission
Currently Reading - Cassini prepares for its final, suicidal mission

A natural color view of Saturn, its rings edge-on and its largest moon, Titan, as viewed from the Cassini spacecraft. Image credit: NASA/JPL/Space Science Institute.

Cassini prepares for its final, suicidal mission

After nearly 20 years in orbit around Saturn, Cassini prepares to say goodbye.


“All the atoms of our bodies will be blown into space in the disintegration of the solar system, to live on forever as mass or energy.” -Carolyn Porco

In 1997, NASA’s Cassini spacecraft was launched for a journey to Saturn, where it would study our Solar System’s ringed world as never before.

Visible and radio images of Saturn’s rings and their structure, as delivered by Cassini. Image credit: NASA/JPL/Space Science Institute.

It delivered beyond our wildest expectations, presenting breathtaking new views of the least dense planet known.

Saturn in eclipse, perhaps the most stunning image of the planet ever taken. Image credit: NASA/JPL/Space Science Institute.

It viewed Saturn in eclipse, discovering two new, outer rings in the process.

An infrared view of Saturn, along with its ring’s shadows on the planet’s atmosphere. Image credit: NASA / JPL / Space Science Institute.

Its infrared eyes viewed Saturn’s hazes beneath the top-level clouds.

A false-color image highlighting Saturn’s hurricane over its north pole, inside the much larger hexagon-shaped feature. Image credit: NASA/JPL-Caltech/SSI.

The north pole of Saturn was found to possess a strange hexagonal storm, thought to be stable over century-long timescales.

Descent Imager/Spectral Radiometer (DISR) image of Titan taken at 2km altitude during the descent. Image credit: ESA/NASA/JPL/University of Arizona.

The Huygens probe released by it descended onto Titan, its largest moon, discovering an incredible landscape, liquid methane lakes and even waterfalls.

Iapetus, the second Saturn moon ever discovered, as imaged by Cassini. Image credit: NASA / JPL-Caltech / Space Science Institute / Cassini.

The mystery of Iapetus, its two-toned moon, was solved as well: dark material from the captured comet, Phoebe, causes the ice on one side to sublimate and settle on the other.

The captured Kuiper Belt object, Phoebe, now one of Saturn’s moons. Image credit: NASA/JPL/Space Science Institute.

Enceladus, an icy, outer moon, was found to contain a subsurface water-ice ocean, which erupts in spectacular geysers.

This is a false-color image of jets (blue areas) in the southern hemisphere of Enceladus taken with the Cassini spacecraft narrow-angle camera on Nov. 27, 2005. Image credit: NASA/JPL/Space Science Institute.

The rings were determined to be made up of 99.9% water-ice, and are at least hundreds of millions of years old.

One of Saturn’s small moons passing in its orbit through a gap in the rings. Image credit: NASA/JPL/Space Science Institute.

And finally, it discovered and viewed the largest storm in the Solar System’s known history: 2011′s Saturnian hurricane.

Image credit: NASA / JPL-Caltech / Space Science Institute, of the great storm’s evolution over a period of 8 months.

Mostly Mute Monday tells the story of a single astronomical phenomenon or object in visuals, images and video in no more than 200 words. Cassini will end its mission in 2017 by crashing into Saturn, thereby avoiding any possible contamination of moons with organics on them.

This post first appeared at Forbes, and is brought to you ad-free by our Patreon supporters. Comment on our forum, & buy our first book: Beyond The Galaxy!

Next Story — Ask Ethan: How Small Can A Piece Of The Universe Be And Still Expand?
Currently Reading - Ask Ethan: How Small Can A Piece Of The Universe Be And Still Expand?

The stars, galaxies and clusters shown here are individually bound together, but do not expand as the Universe does. Image credit: NASA/ESA Hubble Space Telescope.

Ask Ethan: How Small Can A Piece Of The Universe Be And Still Expand?

Galaxy-sized? Human-sized? Atom-sized? Even smaller? How tiny can a bit of space be and still expand?


“We now have the best picture of how galaxies like our own formed their stars.” -Casey Papovich

The expansion of the Universe has a long and amazing history. When Hubble first noticed the relationship between a galaxy’s distance from us and how redshifted its light was, he knew immediately it was a consequence of Einstein’s General Relativity. When Hubble announced his discovery, Einstein immediately recanted his cosmological constant — a “fudge factor” to keep the Universe static — and called it his greatest blunder. But while the space between galaxies expands, atoms, human beings and planets remain the same size over time. What determines this? Jeroen van Rijn wants to know:

What scale limit if any are we talking about when we say the universe expands? Does it mean Planck length isn’t a constant so much? Do atom’s orbits grow corresponding with this stretching of space or does the strong force counteract this?
The “raisin bread” model of the expanding Universe, where relative distances increase as the space (dough) expands. Image credit: NASA / WMAP science team.

The expanding Universe is a tough phenomenon to wrap your head around, because it’s very counterintuitive. Perhaps the best analogy is to imagine that the fabric of space is like a ball of dough, suspended in an oven in zero gravity. As the dough bakes, the bread leavens and rises, and it expands uniformly in all directions. But that’s just for empty space, or space with nothing in it. What if you wanted to have space that contained things like matter: protons, atoms, humans, planets, galaxies or even clusters of galaxies? There are two ways you could imagine the expansion.

The balloon/coin analogy of the expanding Universe. The individual structures (coins) don’t expand, but the distances between them do in an expanding Universe. Image credit: E. Siegel, from his book Beyond The Galaxy.

One is like the surface of a balloon with coins glued to it, where the coins themselves don’t change as the balloon’s surface expands. The Universe gets bigger and bigger, and all the space between the individual particles — or individual galaxies — grows as well. A coin will appear to recede from a nearby coin at a particular rate, while a coin twice as far away will appear to recede at double that rate. The think is that any coin will perceive that same effect: its perceived speed, and hence the redshift (stretching) of the light, will appear to depend solely on the distance of the coin you’re looking at in this expanding space. This much we know happens, and we’ve known it since the 1920s. This was the very same relationship that “Hubble’s Law” demonstrated to us was at play in the Universe.

If everything expanded as the Universe did, then the coins would need to be replaced by paint. Image credit: “Fun with Astronomy” by Mae and Ira Freeman, via http://amzn.to/2aKd9qD.

But the other way you could imagine this is by considering the surface of a balloon with objects painted onto it. As the balloon inflates and its surface expands, it carries the mark of the paint along with it. Sure, the distant objects will all move away from one another in accord with Hubble’s Law, but in this case, the objects themselves would also expand along with the fabric of space.

So what is it that the Universe does? What scale does space expand on? One thing we can do is check the Universe itself. When we look out at the distant galaxies, we should see them redshifted and smaller/lower in mass, as greater distances also mean earlier times.

The evolution of Milky Way-like galaxies throughout different epochs in the Universe. Image credit: NASA, ESA, C. Papovich / Texas A&M University, H. Ferguson / STScI, S. Faber / University of California, Santa Cruz and I. Labbe / Leiden University.

We do see that, but we also see some important other signatures:

  • The galaxies have the same spectral lines at high redshifts, telling us that the sizes and properties of atoms billions of years ago are the same as they are today.
  • That the physical size of the galaxies are determined by their masses alone; that galaxies of the same mass today and at early times are the same physical size.
  • And that the way the cosmic web (and large-scale structure) grows or doesn’t is dependent only on the amount of mass present in a given region of space.
The absorption lines at a variety of redshifts show that the fundamental physics and sizes of atoms have not changed throughout the Universe, even as the light has redshifted due to its expansion. Image credit: NASA, ESA, and A. Feild (STScI).

So it looks like it’s the “coins” analogy rather than the “paint” analogy. When we look out at the Universe, we see that the fabric of space itself expands in all circumstances unless there’s another force that works to bind an object together. This actually fits in theoretically with what we expect completely, because unlike we commonly think of it, the expansion isn’t a force, but is rather a rate. When something becomes bound together, it doesn’t matter what the force doing the binding is, whether it’s a nuclear force in the case of protons and nuclei, whether it’s an electromagnetic force in the case of atoms, cells or humans, or whether it’s a gravitational force in the case of planets, stars, galaxies or even clusters of galaxies.

Wide-field image of the Coma cluster of galaxies. Image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona.

While the fabric of space itself expands, it expands at a certain speed-per-unit-distance. (In reality, this is just units of inverse time.) As a rule-of-thumb, if the force between any two objects causes them to attract at a greater rate than the expansion of the Universe would cause the space between them to expand, then they no longer act like paint; they act like coins. Our bodies are bound together; every atom is bound together; our local group is already bound together; even the entire Coma cluster of galaxies (above) is bound together! But it’s important to remember that this is all relative. The expansion of the Universe doesn’t affect our local group or anything within it because our local group is too tightly bound for that, but go outside of it and space itself continues to expand. This is why distant galaxies (and other bound structures) continue to recede from us, even as individually we’re all locally bound to our own regions of space.

The various groups and clusters we can see here — including our local group — are all individually bound, but the space between each of them is expanding. Image credit: Andrew Z. Colvin under a c.c.a.-s.a.-3.0 license.

But we can go to arbitrarily small regions of space in the unbound regions, where there is no matter present, and we’d find that any sized-region — light years, kilometers, microns, proton-sized, or Planck-sized (or even smaller) — expands in direct accord with Hubble’s Law. The rate of the expansion of space, in General Relativity, allows you to treat the fabric of space as though it’s completely continuous, with no need to quantize it as you might in quantum physics. This remains valid for the expansion of the Universe, right up until the moment where you put a bound structure inside it! There’s no fundamental limit to how small a bit of space can be and still expand, but it needs to either be empty or sufficiently large so that the “structure” that you’re in can’t overcome the expansion itself.


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