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Mosier
01-23-2016, 10:59 AM
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?

Chronos
01-23-2016, 12:21 PM
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.

Zach29
01-23-2016, 01:57 PM
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.

Zach29
01-23-2016, 02:03 PM
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.

Chronos
01-24-2016, 06:28 PM
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.
That direction does not exist. You can no more point towards the outside of a black hole from the inside than you can point towards last Thursday.

And while a closed universe is a form of black hole, the current best evidence suggests that our Universe is not closed.

Carl Pham
01-25-2016, 01:15 AM
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.

Mosier
01-25-2016, 02:50 AM
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.

coremelt
01-25-2016, 03:23 AM
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.

https://en.wikipedia.org/wiki/Timeline_of_the_far_future

MrDibble
01-25-2016, 03:24 AM
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.
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?

Mijin
01-25-2016, 03:56 AM
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)

Asympotically fat
01-25-2016, 01:20 PM
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.

Half Man Half Wit
01-25-2016, 02:23 PM
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 (https://www.cfa.harvard.edu/news/2011-11). But it'd be a mite more challenging than today.

Blue Blistering Barnacle
01-25-2016, 03:33 PM
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...

MrDibble
01-25-2016, 03:53 PM
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.
I hear you.

Chronos
01-25-2016, 05:08 PM
Quoth Blue Blistering Barnacle:

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.
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.

wolfpup
01-25-2016, 09:02 PM
That direction does not exist. You can no more point towards the outside of a black hole from the inside than you can point towards last Thursday.

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.

Blue Blistering Barnacle
01-26-2016, 01:15 PM
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.

But how will you observe the ancient expansion if all you can see is your local gravitationally bound cluster or supercluster?

oh, forget it- you were responding to the second part of my question, I see it now.

Zach29
01-26-2016, 01:45 PM
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.

Zach29
01-26-2016, 02:01 PM
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.

Exapno Mapcase
01-26-2016, 02:05 PM
Look up "not even wrong."

Lemur866
01-26-2016, 02:16 PM
You can also fire a bullet into the sky so fast that it will never fall back down.

And given the observations we've made so far, it looks as if the expansion of our universe has enough energy that it will never fall back down into a big crunch.

Zach29
01-26-2016, 02:29 PM
You can also fire a bullet into the sky so fast that it will never fall back down.

And given the observations we've made so far, it looks as if the expansion of our universe has enough energy that it will never fall back down into a big crunch.
That is not true. Think carefully about it.

Rockets, of course, can change trajectory after initial launch, which is how they can end up in a stable orbit. Maybe that is what you were thinking of.

Zach29
01-26-2016, 02:38 PM
Look up "not even wrong."
Please tell me then where does all this gravitational potential energy reside?

If we know energy cannot be created out of nothing and that matter and energy are equivalent, then all this energy has to exist in the form of mass somewhere right?

Do you have any idea how much gravitational potential energy a single tiny piece of matter has? There are all sorts of gigantic Black Holes in the universe it could fall into.

You might try to argue, "Well, the energy must exist in these Black Holes then", but Black Holes are just a collection of ordinary matter just like our tiny piece of matter in question.

So where is all this mass hiding??
All I am proposing is that this mass is hiding in plain sight: the rest mass of the particle itself.

Do you have another explanation?

Telemark
01-26-2016, 02:46 PM
That is not true. Think carefully about it.

I'm afraid you'll have to explain yourself, because it certainly is true. A bullet fired at greater than escape velocity won't come back down to earth.

Lemur866
01-26-2016, 03:12 PM
That is not true. Think carefully about it.

Rockets, of course, can change trajectory after initial launch, which is how they can end up in a stable orbit. Maybe that is what you were thinking of.

It is true that you can't fire a bullet into the sky and without any course correction have the bullet orbit the Earth. Either it will fall back to Earth or it will leave Earth. It will never orbit Earth.

But it can clearly leave Earth forever.

Zach29
01-27-2016, 02:24 AM
I will point out that you are making the assumption that the Cosmic expansion actually reached escape velocity during the Big Bang. If the energy of expansion was just under the escape velocity, the origin of the universe could still have been able to explode.

btw, it doesn't matter if some of the matter exceeded escape velocity, it is about the average energy of all the matter, because the gravitational force will still be able to eventually pull back all the other matter if the average is less than the full escape velocity value.

Exapno Mapcase
01-27-2016, 12:05 PM
I will point out that you are making the assumption that the Cosmic expansion actually reached escape velocity during the Big Bang. If the energy of expansion was just under the escape velocity, the origin of the universe could still have been able to explode.
During the period of inflation - which is separate from the Big Bang, although often conflated in popular speech - space itself expanded at many times the speed of light. The expanding space pulled matter with it. There was no explosion, therefore. That's a bad term left over from earlier popular attempts to explain the origin.

Escape velocity is also a meaningless term in this case. As you were told in another thread, escape velocity is the speed needed for an object to leave a gravity well and not have it fall back. There was no gravity well involved during inflation. The entire universe inflated simultaneously and everywhere. That's why there is no center to the universe today.

It would be possible for the combined mass in the universe to overcome expansion and retract, what is called the Big Crunch. Big Crunch theories are out of favor these days with the discovery that the rate of expansion is accelerating.

Lemur866
01-27-2016, 12:08 PM
It's not an assumption. It's an observation. Not one made by me, and it may be that we are misinterpreting what we observe.

It could be that we live in a universe that will experience a big crunch. Or it could be that we live in a universe that will continue to expand indefinitely. It turns out that the evidence currently shows that it is very likely that we live in the second kind of universe.

That isn't an assumption. It's just that we now think that the expansion of the universe is actually increasing. If there were going to be a big crunch we should see expansion slowing--just like when you throw a ball in the air it starts out fast and gets slower and slower until it reverses direction and starts to fall back to earth.

The cause of the measured increase in the rate of expansion is given the name "dark energy", which just means "we don't know what it is, but there's a Nobel Prize for the team that figures it out".

Zach29
01-27-2016, 03:20 PM
It's not an assumption. It's an observation.
Logical fallacy, the math does not match your claim. If a projectile is approaching the escape velocity, it can still go into space, it just will eventually fall back down at some point. Mathematically, it's kind of meaningless to say that an object is going at exactly the escape velocity; then it would be theoretically indeterminate whether that object would ever fall back down.

That isn't an assumption. It's just that we now think that the expansion of the universe is actually increasing.
That may just be because space is expanding along with the rest of the matter in the universe.

If there were going to be a big crunch we should see expansion slowing
Maybe it has not slowed yet. Also keep in mind that when we look at redshift of distant points in the universe we are effectively looking backwards in time billions of years. The time scale required to observe any change in expansion within our more local (spatially and temporally) area of the universe would be too long.


The cause of the measured increase in the rate of expansion is given the name "dark energy", which just means "we don't know what it is, but there's a Nobel Prize for the team that figures it out".
Most likely vacuum energy, probably exists as extremely ultra-long wavelength electromagnetic energy pervading space. When the wavelength is so much longer than the scale of the measurement in space and time, it is hard to observe this energy in the conventional sense. Of course this energy exists in equilibrium with other forms of matter/energy. Then there are coherence effects that prevent any of the quanta from individually dumping out their energy to local matter.

Chronos
01-27-2016, 03:45 PM
Escape speed may or may not be a good analogy for a universe without dark energy. But we live in a Universe with dark energy, and it's certainly not a good analogy there.

If you throw a ball up from the Earth at less than escape speed, it'll start off fast, slow down, and eventually stop and come back down. If you throw a ball up at exactly escape speed, it'll start off fast, slow down, and eventually approach arbitrarily close to zero speed, but never quite reach it. If you throw a ball up at greater than escape speed, it'll start off fast, slow down, and eventually approach but not quite reach some nonzero asymptotic speed. But in all three cases, it will slow down.

The Universe is not slowing down. It's as if you threw a ball upwards, and as it climbed, it kept on going faster and faster. Clearly, if the ball is going to do that, it's never going to come back down, no matter what speed you initially threw it at.

Now, the wildcard in this is that we don't really know much at all about dark energy. For all we know, it might some day turn off, or even reverse, and if that happens, we certainly could get a Big Crunch. But by far the simplest assumption, in our ignorance, is that it's just going to keep going forever like it is now.

TeknoRapture
01-27-2016, 04:44 PM
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.


I've always wondered what we can't observe *anymore* because of expansion and other changes in the Universe. Like for example what created Dark Matter. For all we know Dark Matter could be the degenerate matter remnants of a decayed form of matter we can no longer observe.

TeknoRapture
01-27-2016, 04:46 PM
For all we know Dark Matter could be the degenerate matter remnants of a decayed form of matter we can no longer observe.

* Which could make the Universe much older...

Zach29
01-27-2016, 05:08 PM
I've always wondered what we can't observe *anymore*
Yes, because the farther we try to look in the universe, the further back in time we are looking at (because light takes so long to reach us), and at some point looking back in time we no longer see anything. Or rather it is obscured by the cosmic microwave background radiation that resulted from the first moments of the Big Bang. All we can see is an ever expanding edge of all this radiation, which marks the end of the observable universe.