Goodbye, Galaxies!

“I am undecided whether or not the Milky Way is but one of countless others all of which form an entire system. Perhaps the light from these infinitely distant galaxies is so faint that we cannot see them.” -Johann Heinrich Lambert

One of the greatest discoveries of the 20th Century was that many of the great, faint, extended nebulae in the night sky were not merely objects within our own galaxy, like the stars. Rather, these objects were many millions of light-years distant, and were entire galaxies unto themselves.

Image credit: Boren-Simon 2.8-8 ED POWERNEWT Astrograph Image Gallery.

What was even more surprising is that pretty much all of these galaxies were moving away from us incredibly quickly! Not just hundreds, but thousands or tens-of-thousands of kilometers per second, as evidenced by the redshift of their spectral lines.

And what was maybe most remarkable is that when we graphed the distance to each individual galaxy against its apparent recessional velocity, we found a remarkable relationship.

Image credit: James Imamura of the University of Oregon.

It was almost immediately realized that this apparent recessional velocity — inferred because of the observed redshift of light — was very likely not due to galaxies speeding away through space, but rather due to space itself expanding.

How does this work? Every element in the Universe — hydrogen, helium, carbon oxygen, neon, etc. — has its own characteristic absorption and emission lines, characterized by the electron transitions within the atom itself. They are fixed quantities for all such atoms in the Universe, and it means that when you see this characteristic series of lines at a given wavelength, you can infer what elements are present.

Image credit: Serway and Jewett / Thomson / Brooks-Cole, retrieved from cbu.edu.

If this object absorbing these spectral lines is moving towards you, those lines get shifted towards the blue, and if it moves away, they get shifted towards the red, with faster speeds resulting in a more severe shift.

But remember, light itself is simply an electromagnetic wave, defined by a wavelength. And it isn’t just the relative motion between a light source and an observer that can change the wavelength of light; a change in spacetime itself can alter that wavelength as well.

Image credit: Astrobiologist David Koerner, of Northern Arizona University.

And if you imagine space as a rubber sheet, or (perhaps more intuitively) as the surface of a balloon, you can easily visualize what happens if that sheet gets stretched over time. An object that emits light will have its wavelength stretched proportional to how long that light must travel to reach you. The light that travels twice as far travels for twice as long, and hence gets stretched by twice as much.

This natural explanation for Hubble’s Law is a consequence of General Relativity, and leads to our picture of an expanding, cooling Universe that was hotter and denser in the past, but that will be cooler and sparser in the future.

Image credit: Addison Wesley, retrieved from Bryan Méndez at Berkeley.

And as we’ve taken better and better data, we’ve found that Hubble’s Law continues remarkably far into the distant Universe! The expansion rate was faster in the past and has been slowing down, consistent with our predictions from the Big Bang model of the Universe. And when we set the Hubble Space Telescope onto the task of measuring what the expansion rate of the Universe is today, we found that it was 72 km/s/Mpc, meaning that for every Megaparsec away from us (about 3 million light years), a typical galaxy speeds away from us at 72 km/s.

Image credit: Freedman et al. (2001), results from the HST Key Project.

But since those results have come out, we’ve seen farther into the distant Universe. And seeing farther is incredibly interesting, because the farther away you look, the longer the light has been traveling, and hence, effectively, the farther back in time you’re seeing.

And you can measure more precisely — and over a longer baseline of time — exactly how the Universe’s expansion rate has been changing. Based on this, because we understand the laws of physics, we can determine the fate of the Unvierse!

Image credit: Addison Wesley, again retrieved from Bryan at Berkeley.

And what we’ve found, as you can see below, is that the Universe isn’t set to recollapse, nor is it even set to be “critical,” where the expansion rate continues to drop, and asymptotes to zero. Instead, our best data indicates that we live in an accelerating Universe!

Image credit: Williamson et al. (2007), using Supernova Cosmology Project data.

In an accelerating Universe, you can imagine three separate possibilities, once again, for the fate of it all. Perhaps the acceleration will be uniform, perhaps the acceleration will slow and reverse itself, leading to a recollapse after all, and perhaps the acceleration itself will continue to rise and speed up, ripping the fabric of the Universe apart!

Image credit: Flickr, originally found at Mysterious Universe.

Well, you know me, and I don’t like to speculate; I like to look at the data and see what it indicates! And what it shows, very clearly and unambiguously, is that dark energy — and hence the accelerated expansion — is incredibly, perfectly consistent with a constant.

What does this mean for the future of the expanding Universe? What does it mean for the expansion rate? It means that the expansion rate will continue to drop, but not by that much more. Currently at around 72 km/s/Mpc, the rate will eventually drop and asymtote down to about 60 km/s/Mpc, but no lower than that. Even though the expansion rate is still going down, the expansion itself is still accelerating! How is this possible?

Image credit: Adam Riess, hubblesite.org, retrieved from snap.lbl.gov.

Imagine you’ve got a galaxy that’s 10 Mpc away from you, and the Universe is expanding at a constant rate of 60 km/s/Mpc.

  • Initially, this galaxy appears to speed away from you at 600 km/s. Over time, it moves farther and farther away from you, and eventually, it will be twice as far — 20 Mpc — as it was initially.
  • But once the galaxy is 20 Mpc away, the expansion rate is still 60 km/s/Mpc. So at 20 Mpc away, the light from it is redshifted as though it moved away from you at 1,200 km/s.
  • And this continues; when it’s 100 Mpc away, it appears to move away at 6,000 km/s. When it’s 1,000 Mpc away, it appears to speed off at 60,000 km/s. In other words, even though the expansion rate doesn’t increase, the speed of any individual galaxy increases over time!
  • By time a galaxy gets more than 5,000 Mpc away from you, you’ll probably be worried. Because those galaxies will appear to speed off at speeds exceeding 300,000 km/s, which is the speed of light!

What, then, happens at that point?

Image credit: Don Dixon, in Scientific American.

The galaxies, once they achieve that distance from us, can no longer send us light! The space of the Universe expands too much and too quickly for light to ever reach us from a distant galaxy!

What will this mean for the galaxies in our Universe today? The ones we see peering out into the abyss of deep space?

Video credit: NASA, ESA and G. Bacon (STScI), originally from here, available on YouTube.

Yes, they’re beautiful today. But tens of billions of years from now? They’ll all be gone. In fact, if we were born a hundred billion years from now, we would have merged with the Andromeda galaxy and (very likely) the rest of our local group, and thanks to dark energy, there would be no other galaxies or clusters visible anywhere in the Universe!

We are fortunate to live now, as we do, or we may never have discovered the rich galactic structure in our visible Universe, because it will all accelerate away!

Image credit: NASA / Hubble Space Telescope / WFPC2, retrieved from hubblesite.org.

So the next time you hear anything about a distant galaxy, no matter where or when, make sure you appreciate it all the more, because billions of years from now, no one in our galaxy will ever know that it exists.

Comments

  1. #1 AngelGabriel
    December 15, 2011

    It’s a nice summary but… it drifts into the rabidly philosophical when you say things like “In fact, if we were born a hundred billion years from now..”

    Is this a fact or a prediction or waxing philosophic or poetic or turning theory into dogma. If a prediction then say “In prediction..” and give some margin of error or such.

    You recently said, “I’d bet you even money that they confirm a Higgs somewhere around 126 GeV by the end of 2013.” Now that’s a prediction that even Jimmy the Greek can understand.

    But when you “predict”, “But tens of billions of years from now? They’ll all be gone. In fact, if we were born a hundred billion years from now, we would have merged with the Andromeda galaxy and (very likely) the rest of our local group, and thanks to dark energy, there would be no other galaxies or clusters visible anywhere in the Universe!”; Well that’s a prediciton that Bernie Maddoff can understand. It’s a scientific ponzi scheme. Predicting infinities and trends that go on forever is not science. such “predictions” ignors the limits of knowledge in accuracy,precision, and new understanding.

    We might as well “predict” that when there are “no other galaxies or clusters visible anywhere in the Universe”; then God will dsecend from the heavens and say, “good job mankind”.

    “100 billion year” “predictions is just sloppy science.

  2. #2 David Marjanović
    December 15, 2011

    And what we’ve found, as you can see below, is that the Universe isn’t set to recollapse, nor is it even set to be “critical,” where the expansion rate continues to drop, and asymptotes to zero. Instead, our best data indicates that we live in an accelerating Universe!

    Uh, yes, but then why does the picture above that paragraph show that, given the age of the universe (13.72 billion years), it’s expected to lie between constant (14.4 Ga or so) and critical (10 Ga), not in the acceleration zone? Are the graphs simply wrong?

    We might as well “predict” that when there are “no other galaxies or clusters visible anywhere in the Universe”; then God will dsecend from the heavens and say, “good job mankind”.

    Well, yeah, but that’s a lot less parsimonious.

  3. #3 Neil Batea
    December 15, 2011

    Note that if space is flat or hyperbolic, it is literally infinite in extent: there are then copies of us, of any event no matter how low probability, indeed an infinity (aleph null) of them! As for space expanding, it couldn’t then refer to the literal size of a hyperbubble. Instead, it means that the relations among things are not like stuff blasting away at various speeds “into space” (but didn’t Milne try to get away with that? – his weird model wouldn’t allow for appropriate gravitational interactions, deceleration etc. AFAICT.)

  4. #4 Neil Bates
    December 15, 2011

    Pardon previous typing error in my name, must slow down! AngelGabriel, IMHO your criticism is unfair. The predictions here are based on the best theoretical constructs we have, whether anyone is around to see them or not. I think you’re too conservative (in the relevant context) a thinker.

  5. #5 dean
    December 15, 2011

    it is literally infinite in extent: there are then copies of us, of any event no matter how low probability, indeed an infinity (aleph null) of them!

    I can’t quite see the link between an infinite universe and assurance that there would be copies of us. Would it be equally reasonable to say that there would be “infinite” (abuse of the word, I know) variety throughout? I admit to being (only) a statistician and not a physicist, so perhaps there is something I’m missing?

  6. #6 OKThen
    December 15, 2011

    “By time a galaxy gets more than 5,000 Mpc away… (it)will appear to speed off at speeds exceeding 300,000 km/s, which is the speed of light! What, then, happens at that point? The galaxies, once they achieve that distance from us, can no longer send us light! The space of the Universe expands too much and too quickly for light to ever reach us from a distant galaxy!”

    Of course 5,000 Mpc is exactly 13.7 billion lightyears (give or take a smidgeon).

    So the observable universe has a visible radius of 13.7 billion lightyears NOW (due to big bang) and FOREVER (due to accelerated expansion). Explain that coincidence please.

    Furthermore, you say “galaxies will APPEAR to speed off at speeds exceeding 300,000 km/s, which is the speed of light!” I’ve capitalized the word “APPEAR”.

    Now why do FUTURE redshifts indicate apparent velocities; but PAST redshifts (i.e. big bang) indicate actual velocities. Note: Edwin Hubble preferred always to talk of astronomical recession velocities (i.e. redshifts) as “apparent velocities”.

    Why would a general relativity theory of the universe (i.e. a Friedmann–Lemaître–Robertson–Walker universe) ever calculate a velocity greater than the speed of light for any currently observed galaxy. Because as you’ve recently said, “So whatever the verdict winds up being with these neutrinos, rest assured that all the protons and electrons that make up you and the world you love are still bound by the laws of special relativity.” ditto galaxies! yes, yes?!

    So unless I’ve been sleeping (which is possible); I thought the inflationary hypothesis was consistent with general relativity and special relativity. Or does the inflationary hypothesis modify either special relativity or general relativity? Or does the observable universe disagree with special and/or general relativity?

    Someone please clarify. Thanks.

  7. #7 Doug Little
    December 15, 2011

    So whatever the verdict winds up being with these neutrinos, rest assured that all the protons and electrons that make up you and the world you love are still bound by the laws of special relativity.” ditto galaxies! yes, yes?!

    Well the particles/galaxies themselves aren’t travelling faster than light it’s the expansion of space in between that makes them recede faster than light, actually the galaxies could be moving toward us it’s just that the space in between is expanding faster. As far as I know this concept of warping space is one of the more viable ways to travel ftl.

  8. #8 The Peak Oil Poet
    December 15, 2011

    I know this will seem silly

    but really, the idea of an expanding universe seems to illogical – it just feels wrong

    why can there not be another explanation for all the observed phenomena?

    what observed phenomena can not be explained by an expanding universe or, what hoops have had to be postulated so such phenomena could be forced into the model?

    pop

  9. #9 Duncan Ivry
    December 15, 2011

    The Peak Oil Poet: “why can there not be another explanation for all the observed phenomena?”

    I don’t think your comment is silly, and, of course, there *can* be another explanation, but for some observations no other explanation has been proposed at all, and for some observations no other *better* explanation has been proposed. Ethan already said a lot about these explanations in previous articles.

  10. #10 thequiet1
    December 15, 2011

    “So the observable universe has a visible radius of 13.7 billion lightyears NOW (due to big bang) and FOREVER (due to accelerated expansion). Explain that coincidence please.”

    It’s not a coincidence, it’s causative. The radius of the observable universe is exactly what it is because anything beyond that distance appears to move away faster than the speed of light, and hence is unobservable. The total universe is thought to be much, much larger than what we can observe.
    Also, the visible universe actually has a radius of about 46 billion light years. To understand why have a read of http://en.wikipedia.org/wiki/Observable_universe

    “Now why do FUTURE redshifts indicate apparent velocities; but PAST redshifts (i.e. big bang) indicate actual velocities. Note: Edwin Hubble preferred always to talk of astronomical recession velocities (i.e. redshifts) as “apparent velocities”.”

    When someone refers to ‘apparent motion’ they are referring to something which appears to move relative to us because of the expansion of space, but which isn’t actually moving through(/i> space.

    “Or does the observable universe disagree with special and/or general relativity?”

    This is something that stumped me once too. The answer is that the speed of light is only a limit for things moving through space. Space itself can expand as fast as it wants.

    The astronomycast podcast addresses all of these questions in different episodes and really helped me grasp a lot of these concepts. I highly recommend it.

  11. #11 thequiet1
    December 15, 2011

    “why can there not be another explanation for all the observed phenomena?”

    As Duncan says, there can be alternatives. If anyone comes up with a better fit for our observations than the current model, we will replace it. That’s how science works.

    The thing is, big bang theory doesn’t just explain what we have observed, it made several predictions which have been fulfilled. Ethan has written many entries about this topic, like this one:

    http://scienceblogs.com/startswithabang/2010/04/big_bang_alternatives_anyone.php

  12. #12 Hermit
    December 16, 2011

    While reading this post something popped in my head. Between the speed of the expanding universe and the distances we can observe in that expanding bubble, mabye just mabye. Our “Big Bang” wasnt the first. Let me explain my thoughts. Eventualy we will not be able to see anything else in the surrounding universe except out own galaxy. But at some point we will merge with Andromeda. How many other galaxys will we absorb or at leaast siphon off matter from? Could we eventyaly amass so much that the “Andromeda Way” collapses in upon itself igniting a “Big Bang”. What if this has been happening all along and we are just another firecracker waiting for the fuse to burn down to our turn? Then the cycle starts all over agin.

  13. #13 Ilkka M
    December 18, 2011

    You can’t move faster than light, so how can entire galaxies move faster than light?

  14. #14 Duncan Ivry
    December 18, 2011

    Ilkka M: “You can’t move faster than light, so how can entire galaxies move faster than light?”

    Would you read Ethans article and the comments above again, please? What you are asking has to do with, on the one hand, the movement of galaxis inside and through our universe, and, on the other hand, the expansion of space, taking galaxis with it. The speed of the latter one is not limited by the speed of light.

  15. #15 Ilkka M
    December 19, 2011

    Ok, thank you Duncan, now i get it!

  16. #16 OKThen
    December 28, 2011

    @14 Duncan
    You say, “the expansion of space, taking galaxis with it. The speed of the latter one is not limited by the speed of light.” And that is the neat but questionable trick of the inflationary hypothesis, which I doubt and don’t wish to discuss here.

    But if we accept that the universe is expanding due to dark energy; and if we take the simplest case where dark energy is like the cosmological constant of general relativity; and since general relativity was constructed to agree with special relativity; then why (for the expansion of the universe due to dark energy) are velocities greater than the speed of light c ever achieved?

    And if galaxies can be acted upon to achieve velocities greater than c; then why can’t that same force act upon neutrinos, atoms, you me etc?

    Now finally, just for dark energy accellerated expansion: does such expansion to velocities greater than c violate general relativity. Please point to to a reference in a general relativity textbook (title page etc) that discusses v>c as in agreement with general relativity and special relativity.

  17. #17 marcc777
    Canada
    January 12, 2014

    @OKThen

    I know that this is an old post but I must say that if you decide to reject the current explanation which is generally accepted by the scientific community and say that the inflationary hypothesis is wrong about the inflation of space then how can you expect someone to answer your question? That would be like me asking you what color said object is and when you answer that it is red I accuse you of being wrong and then immediately ask you the same question again. If you disagree with the currently accepted explanation then you must know better than the rest of us and rather than asking questions should be giving the answers. What is your reason for disagreeing? Where is your better explanation?