…and the rate of expansion is increasing, will our galaxy ever reach the speed of light? If so, how would that effect us?
The evidence that the universe’s expansion rate is increasing is far from conclusive. However, no, no matter can reach the speed of light. And long before any changes are noticeable for us, our solar system and probably our galaxy will no longer exist in a recognisable form.
Regardless of whether or not the Universe is accelerating (and my impression from hearing the cosmologists around here talk is that the evidence isn’t quite as inconclusive as GorillaMan implied), the expansion of the Universe doesn’t actually make any material objects accelerate. Rather, it’s the space between the objects that changes with time. It’s a fine distinction, I’ll grant you, but it’s an important one.
In other words, if I’m sitting on Galaxy A and you’re sitting on Galaxy B, we’ll each see the distance between us increase; and if the Universe’s expansion is accelerating, we could both interpret this as “the other guy is accelerating away from me.” It’s even possible that the expansion of the Universe will get sufficiently fast that the space between us will be expanding faster than the light can cross it, in which case we might think that the other guy has started travelling faster than the speed of light. However, neither one of us will have actually accelerated; in our respective frames, we’re both sitting perfectly still. It’s the behaviour of spacetime itself that makes it appear that we’re moving away from each other.
Ain’t General Relativity great?
Maybe using the old “rubber sheet” analogy would help:
Take a rubber sheet, that’s space-time. Put some objects on it, those are galaxies/planets/atoms/whatever. For simplicity, let’s assume everything is “at rest” relative to it’s little neighborhood.
Relativity says that the objects can’t move across the sheet faster than light. But the universe’s expansion is just stretching the rubber, the objects themselves aren’t moving relative to their local chunk of sheet. If the sheet was stretching fast enough, objects could be going “faster than light” relative to us, but that really means that the sheet is adding more distance faster than a beam of light can traverse it, making those objects effectively invisible (and unreachable).
It’s pretty darn conclusive. Both the SnIa data collected by the Hubble and the WMAP data (completely independent, and using very different methods) agree very closely on the same value for the acceleration, and there are several other more circumstantial results which also support it. There is still a good bit of debate on the behaviour of the acceleration (whether the acceleration changes in time, what causes it, etc.), but all cosmologists are pretty much forced to accept that it exists, now.
If the acceleration continues to behave in the manner it appears to be doing so now, then galaxies and clusters of galaxies will continue to remain together forever. There’s some speculation that the acceleration might be increasing, such that eventually galaxies, solar systems, and even atoms would be pulled apart in a “Big Rip”, but current evidence seems to be against this possibility.
So how exactly did they determine that empty space is expanding? Is this somehow related to dark matter?
Basically, by the Doppler effect. This page has a good explanation. We see objects further away from us redshifting–that is, the wavelengths are increasing. You can hear the same thing next time something with a siren goes by. The sounds will appear to get faster as the car approaches and then slower as it gets further away. Of course, it’s more difficult in astronomy because you have to take things like relativity into account, but that’s a good basic idea.
I’m sorry. I should have said that the pitch will get higher as it approaches you and lower as it gets further away from you.
I understand the idea behind redshifting, but I’m still a little confused.
If a star is redshifting, that means it is getting farther away from us, whether it is actually moving away or the space between it and us is getting bigger, making it seem that it is moving away. To borrow the rubber sheet analogy from earlier, I still don’t get how they decided that the rubber sheet is being stretched as opposed to the objects on it rolling away.
Also, if it’s the empty space which is expanding all around us, then why would we see some objects blueshifting, or appearing to get closer to us?
The expansion of the universe was discovered by Edwin Hubble, the guy they named the space telescope after. He first established that spiral galaxies were not part of our own galaxy. Then he took spectra of those galaxies and found that, with a few exceptions, all galaxies are redshifted. Further, he found the dimmer the galaxy (and thus the further away it was) the bigger the redshift.
I don’t know all the details (since I’ve never studied General Relativity), but, as I understand it, one solution to the GR equations has the universe expanding. So Hubble’s results seemed to match this. (I’ll let Chronos or someone else discuss the Cosmological Constant; I’m not sure I could do it justice.)
All the objects that are blueshifted are local, either stars within our galaxy or among the local group of galaxies. These are all gravitationally bound together and thus the expansion of the universe does not cause them to recede from each other (including us, of course). The Andromeda galaxy is blueshifted, for example, as are about half of all stars.
As dtilque explained, Hubble discovered that distant galaxies were moving away from us, and the relationship between their recessional velocity and their distance was an exact proportionality, i.e. velocity = distance * a constant, later named the Hubble constant in his honor. Now, we don’t know that there isn’t some Vast Cosmic Conspiracy[sup]TM[/sup], and we here in the Local Cluster happen to inhabit the only place in the Universe that all the galaxies are moving away from. (Who knows? Maybe we’ve got galactic BO or something.) However, ever since the days of Copernicus, physicists have been very uneasy about saying that the Earth occupies a privileged place in the Universe. The fact that the “expanding space” model doesn’t require any privileged spatial location at all (in fact, the solutions of GR that dtilque alluded to are usually derived by making the assumption that no such location exists) means that physicists are a lot more philosophically inclined to believe it.
I suppose that if you don’t buy the argument that there’s no reason for the Earth to be a special place, cosmically speaking, then the only way to directly test the “expanding sheet” model as opposed to the VCC[sup]TM[/sup] model would be to go to another galaxy that’s far enough away not to be gravitationally bound to us — somewhere in the Centaurus Supercluster would probably do — and see if Hubble’s Law still holds. However, that’s 140 million light-years away, so if you want to go there & do the observations yourself, you’d better pack a lunch or something.
Okay, this is making more sense now. See, this is what happens when a layman tries to read Stephen Hawking.
Thanks all.
The two are distinguishable. To use an extreme case: Suppose that we have two galaxies, A and B, in a universe which is initially static (neither expanding nor contracting). A beam of light leaves A, headed for B. Then, while the light is en route, the universe suddenly starts expanding, for whatever reason. It expands by a factor of two, all while that light is travelling, and then stops expanding and becomes static again (but at twice the size). Finally, the light reaches galaxy B. Now, if the galaxies were just moving apart within space, and the redshift were just a true Doppler shift, the light detected at galaxy B would not be shifted at all. However, if the separation of the galaxies is due to the space itself expanding, then the light will be expanded along with space, and it will be redshifted by a factor of two.
Now, of course, the real Universe isn’t nearly this cooperative, but there are still observational differences between a Doppler redshift and an expansion-of-space cosmological redshift.
As for the further historical question: It was originally believed (up until the early 20th century) among scientists that the Universe was essentially static and unchanging. With Einstein’s General Relativity, though, we finally had a theory of gravity capable of discussing the Universe as a whole, and he found that a static universe was not allowed under his theory. So he added a slight modification, which he called the Cosmological Constant, which basically acted as a repulsive gravitational force over large distances. If you give the Cosmological Constant just the right value, then you can balance normal, attractive gravity with this repulsive force, and get a static universe.
But not a stable universe. The C. C. force gets stronger with distance, whereas ordinary gravity gets weaker with distance. So if some perturbation were to cause such a universe to expand ever so slightly, the C. C. force would get a little bit stronger, and attractive gravity would get a little bit weaker, which would cause the universe to expand more, and so on. Likewise, if it were perturbed to be a little smaller, attractive gravity would gain an edge, and eventually take over.
And then Hubble discovered the expansion of the Universe. This was perfectly consistent with GR without the Cosmological Constant: The Universe, presumably, is being pulled inwards by gravity and therefore slowing down, but it started off expanding and just hasn’t slowed to a stop yet (and might never: It was not known whether the Universe was expanding at its “escape speed” or not, and it could theoretically asymptotically approach stopping, or asymptotically approach a constant rate of expansion). When Einstein heard about this discovery, he immediately discarded the notion of the Cosmological Constant, calling it the biggest blunder of his carreer.
Sometime in (I believe) the 1980s, though, some cosmologists began to have second thoughts about this. Evidence available at that time, if interpreted without a cosmological constant, seemed to indicate that the Universe itself was younger than the oldest stars in the Universe. One possible solution would be the existance of a nonzero Cosmological Constant, which would make the Universe appear younger than it actually is. But there was no direct evidence for such a beast.
Enter the other Hubble, the Space Telescope. The primary mission of the Hubble Space Telescope was to measure the distances and redshifts to many distant galaxies (of course, the HST also served many other purposes very well, but that was the primary one). The goal was to determine just how much the Universe is decelerating, to indicate whether or not it would expand forever. Much to everyone’s surprise, though (well, not everyone’s, but most folks’), it turned out that the Universe isn’t decelerating at all. It’s accelerating, expanding faster as time goes on. This is inconsistent with plain, vanilla GR: We need something like the Cosmological constant to explain this. A couple of years ago, the MAP satellite, observing the microwave background radiation (the leftover heat from the Big Bang) provided a completely different source of data, which leads to the same conclusion: We have a cosmological constant, bigger than Einstein thought, and which is now the dominant force at work in the Universe on the largest scales.
Note, again, that the C. C. is not necessary to explain an expanding universe, but it is necessary to explain our Universe which is not only expanding but increasing its rate of expansion.
As I understand it, there are two different theories concerning this acceleration: dark energy and modification of the laws of gravity due to hidden dimensions.
If the presumed dark energy is constant, then this is Einstein’s c c: an unchanging property of empty space that imbues the universe with a constant acceleration. But if it’s not constant, it could either increase in strength and rip the universe apart, or it could fade away, eventually causing the cosmos to collapse.
The other theory involves extra, hidden dimensions. This theory holds that the universe is confined to a 4-dimensional space-time, called a “brane” (short for membrane), that’s embedded in a higher-dimensional “world.” Because gravity is an intrinsic property of all of space-time, it may be the only component of the cosmos that isn’t trapped on this 4-dimensional brane. When gravitons, the particles that mediate gravitational attraction, escapes our brane, the gravitational force that remains within the brane diminishes. Leaky gravity behaves like dark energy.
I thought there was some sort of measurement made recently that might put a damper on the brane theory. I can’t remember what it was; it seems like it had something to do with the measurement of something’s speed.
RR
Nothing more to add, except we are now discovering galaxies w/redshift = 7, and have closed inside of 1 billion years before the Big Bang.
Could you elaborate on this a bit more? I had been under the impression that the observed redshift of galaxies could either be the result of the expansion of the universe or some kind of large explosion with us at the center. In the latter case, the galaxies that were initially given a greater velocity would be further away by now, which is consistent with Hubble’s Law. If there is a purely observational difference between these two scenarios, it escapes me.
So, there doesn’t “need” to be a center of expansion? But surely there “must” be a point in space that doesn’t move at all, relative to the other spots. (Not necessarily us, of course. But I’m not well-versed enough in 3-D space to construct a shape that would allow for all particles to think that all other particles are moving away from it when expansion occurs.)
Ah, yes, now I remember why I didn’t take astrophysics…
Have you ever made bread? As the yeast in the dough produces CO[sub]2[/sub] the flour particles are pushed apart. Because CO[sub]2[/sub] is produced uniformly throughout the loaf, the speed at which any two bits of flour move away from each other is directly proportional to the distance between them. In other words, there’s a Hubble constant for rising bread just as there is for the expanding universe.
Now if you’re looking to find the center of your loaf of bread, the easiest way is to just measure it with a ruler. However, there’s no way that you can deduce where the center is, or even if a center exists, just by making a bunch of measurements of how far apart and how fast the various parts of the loaf are receding from each other.
<mode babble=“1”>The “large explosion” scenario doesn’t require us to be in the center of everything for it to appear like everything is moving away from us.
Take a set of points, A, B, C, D, E, F, etc. A is the center of the explosion, with B moving away at X, C at X+, D, at X++, E at X+++, etc. From D’s frame of reference, A is moving away at speed X++. C and E are moving away from D at +, F and B are moving away from D at ++, etc. Of course, if that were the case, we’d expect to see a ring of galaxies with very low relative velocity representing those roughly the same distance away from the center of the explosion, spreading apart due to spherical expansion of the blast radius. Maybe that’s what we call the “local cluster”?</mode>