Lawrence M. Krauss wrote a tremendous (in my opinion) book called A Universe From Nothing, which pretty neatly explains how stuff exists, and how “nothing” gives rise to everything.
Part of the book is dedicated to explaining the expansion of space, and what that means for us and far-distant future astronomers. For us, it means that we live in precisely the right moment to observe that this expansion is actually happening, but for the future astronomers that situation is more bleak.
Krauss predicts that eventually, the universe will recede so far and fast that only a single galaxy cluster will remain, causally isolated from everything else. Today, we know the universe is expanding by observing the receding galaxies far away from us, while this hypothetical future alien astronomer can make no such observations. It is therefore virtually impossible for this future astronomer to create an accurate model of the universe which includes expansion. This astronomer will never even know he is wrong, as there would be no prediction or experiment he could make, the results of which which could confirm expansion.
My questions is, so what? This future astronomer wouldn’t necessarily be wrong. In fact, if there’s absolutely no evidence (from his perspective) that the universe is expanding, no phenomenon that could be better explained by an expanding universe than a static one, no accurate prediction that an expanding universe would make better than another explanation could,…wouldn’t it be perfectly reasonable (and even accurate) to say that space is NOT expanding, from the perspective of this hypothetical astronomer? Wouldn’t he actually be correct to infer a static (or maybe even collapsing?) universe? In what sense would his conclusion be wrong?
I think this is more properly a question of philosophy than of science. It boils down to asking just what the task of science is: To determine what really is, or to determine what is observable?
That said, however, said far-future scientist could come across archaeological remains of scientists from our own day, who left behind our own records showing the expansion of space. It’s also possible that some “natural” (meaning requiring no input from intelligent beings) processes might also leave behind records of some sort that would contain a signature of expansion.
If the universe originated from a single point, then that point had to be a pure singularity of energy, effectively a standing wave with a very small wavelength. Wavelength is inversely proportional to energy, so the higher the energy of a wave, the shorter the wavelength. We are basically talking about a single photon, but at such high energies this photon would exist in equilibrium with many other particle manifestations of energy.
It is possible to view the entire collective universe as a giant Black hole in a way. Because as electromagnetic energy continues to radiate outward it is very slowly losing energy and increasing in wavelength. There is something called the critical angle (which is never precisely zero) so this radiation can never actually just disappear. Eventually at some distant point in time it will begin to boomerang back. But it will have a very large wavelength at the point it is farthest away it can get, and its energy will most likely merge with the quantum vacuum (which pervades all the seemingly empty space) at that point.
Electromagnetic radiation can escape from a singularity Black hole. Because the smaller the diameter of the Black hole, the greater the critical angle can be for the radiation to escape. To say it more clearly: despite popular perceptions, electromagnetic energy CAN escape from a black hole, as long as it is escaping along a vector that is precisely perpendicular to the surface of the event horizon.
This is normally not a considerable effect with a regular Black hole, but when we are talking about the singularity from which the universe started from, the phenomena would manifest itself very differently because of the extremely small size.
He could still measure the cosmic background radiation, and assuming he had duplicated general relativity (and general relativity is correct), that should allow him to both deduce that the universe is expanding and confirm at least some of his predictions by experiment.
That was brought up in the book. The CBR in an expanding universe will eventually become so diluted that it will be undetectable to even the most sensitive theoretical instruments.
What time scale are we talking about where our galaxy would be the only one we can see because the rest have moved beyond the observable universe?
I suspect our hypothetical future astronomer would bigger problems than not being able to work out the universe is expanding. The timeline of the far future page on wikipedia makes fascinating reading. Its possible that the Big Rip will end the universe in a mere 22 billion years. And star formation ceases in 1 trillion years.
IANAP, but by that point, wouldn’t the recession of individual stars in the observer’s single home galaxy become observable? I mean, the universe will still be expanding at all levels of observation, right?
I don’t think so; gravity locks together the material in a galaxy, and if anything they become a little more tightly bundled over time.
That said, is that another kind of observation future scientist could make: that objects are accelerating towards each other very slightly less than would be expected, and the degree of error increases with distance.
(IAANAP)
I think the point I would make is that there are always observations an observer could potentially make to see that the Universe is expanding, it’s just that these observations might need long time scales, very sensitive instruments or the luck to be in the right place at the right time. Therefore I would say it certainly isn’t just like the Universe isn’t expansion, it’s just that the Universe’s expansion is considerably easier to observe in the current epoch.
The turbulent dynamics in the galactic core occasionally lead to the ejection of hypervelocity stars that are fast enough to leave the galaxy; observing them then allows to deduce the universe’s expansion. But it’d be a mite more challenging than today.
Isn’t it possible that a more complete model (including expansion and dark energy) available to current-time physicists might point the way to physics we don’t know yet (I just know warp drive is only 20 years away! ). Then it *might *not be possible to make those inferences if you live in an epoch where you cannot detect cosmic expansion. Or you might have a theory that included expansion, but you might have no means to favor that.
For example, IIRC, general relativity does not prefer a static universe, but predicts an expanding or contracting universe. But if you live in an epoch where all you have to observe is your own gravitationally bound cluster, you might suppose that the universe is expanding or contracting, but you’ll have a lot of trouble telling which. But you’d still miss dark energy (unless it’s the kind that causes ‘Big Rip’).
BTW, I suppose if you live in a universe fated to expand and then contract, physicists living in the epoch near the balance point would see a ‘static’ universe.
Kinda makes you wonder if there are subtle points of physics that would be easier to detect in earlier or later epochs than ours…
Not exactly, because the observations are not all made at a single point in time. More distant observations are of the Universe as it was at the corresponding time in the past, and even if those things aren’t expanding now, you’d still see signs that they were expanding then. To be sure, though, it would be a lot less noticeable than the present expansion.
As I understand it, that’s quite literally true. A straightforward interpretation of the Schwarzchild solution to the field equations describing the curvature of spacetime yields the result that if you are beyond the event horizon – your distance from the singularity r < the Schwarzchild radius – then the t coordinate and the r coordinate change places. The direction “out” of the black hole is your past, the singularity becomes your future, and escaping your inevitable fate is just like trying to go back to yesterday or avoiding tomorrow.
The following is not established theory among the scientific community, but I had the idea that if all that matter in the entire universe were confined to a single point it would exist as pure energy. The matter was only created as this energy overcame the intense gravitational forces holding it together. Because when you create potential energy, the mass from that energy has to reside somewhere. If or when all the matter in the universe falls back together into a single a point, all that mass will begin to turn back to energy again. As matter falls into a single region, its rest mass will be converted to energy. The particles still exist, they just have less rest mass (actually I think it think it would be more complicated because they might only have less rest mass relative to an outside frame of reference that was removed from the localized effects of the intense gravity, not sure).
Also, it is not only matter/energy that falls into the single point but space along with it. Seemingly “empty space”. That is why theorists say that space expanded along with the explosion of all the matter in the universe. How can this be? Well first you have to realize that space actually as its own energy, although it is not well understood at this time. In the absence of any of this “space”, a particle could move instantaneously from one point to another unhindered. The concepts of time and distance would be meaningless. But in the presence of background energy, the particle’s mass becomes coupled to the energy pervading space. All the space and mass in that exist in the universe are inextricably linked. The “Big Bang” created space at the same time it created all the matter, many people do not understand this.
If you fire a bullet up towards the sky, it will eventually fall back down to earth going just as fast as when it left to barrel of the gun (let’s assume there is no air resistance). That is why it can be so dangerous to fire a gun up into the air, because the bullet is going to come back down and could severely hurt or even kill someone. In some areas where people illegally fire off guns in cities during holiday celebrations, bullets have been known to come crashing down through the roofs of houses.
Think about it. The velocity of the bullet is gradually converted into potential energy as it travels upwards and overcomes the force of gravity. Then this potential energy is eventually converted back into velocity and the bullet falls back downwards from great heights.
It is the same thing with Cosmic expansion. A huge amount of energy was converted into potential energy overcoming the great gravitational forces. But on the other hand, matter did not have as much rest mass at this point in time, so it would have been easier to expand.
This may be hard to understand: Why did matter have less rest mass? Because there is a fundamental link between mass and gravity. When matter falls into a gravitational well, some of the mass of that matter is converted to energy. This is very intuitive.
). Then it *might *not be possible to make those inferences if you live in an epoch where you cannot detect cosmic expansion. Or you might have a theory that included expansion, but you might have no means to favor that.