Are there any infinities in the universe? Could there be?

Great Debates
141

Yes, that is exactly what that means. Well, not because it is redshifted, but because of what caused it to be that redshifted, as I explained in the unquoted second half of that sentence.

That is not relevant.

Once something has passed beyond the cosmic horizon, it is effectively the same as though it had passed through the event horizon of a black hole. Their future light cones will never again cross.

Eventually, in any casually connected patch of space, you will either have one photon that is massively redshifted, or you will have nothing at all. Most, and an increasing number of those patches will have nothing.

Or of course, you have the quantum fluctuations that give rise to a whole new universe.

True, but I’m pretty sure that “New Coke” only ever existed in ours.

142

That’s how we know that this is the darkest timeline

143

Yes, but what does it look like to me, the observer? As a light-emitting object approaches the particle horizon, due to the expansion of space, the light waves are redshifted more and more. To me the time dilation becomes extreme as the object approaches the particle horizon. My point is that I never observe it actually pass the horizon - I can never observe it leave. It will just appear to slow down for all of eternity, never stopping, never leaving, always dimmer and more redshifted.

~Max

144

If a star is slipping past this event horizon, you will see fewer and fewer photons that are more and more redshifted coming from it, until no photons reach you at all.

145

You can’t observe a photon unless you actually interact with it.

146

The object would be a star or something, the photons would be coming directly towards me at the speed of light.

~Max

147

Yes, but eventually the distance between you and the star is growing faster than the speed of light (because space is growing, NOT because the star travels through space), at which point photons shooting towards you will never reach you.

148

I guess there’s some part of quantum mechanics where a light emitting body emits a finite number of photons at a known rate? Because if we’re treating it as a wave the quantity never goes down to exact zero.

How would it work if some object physically moved out of range while emitting gravitational waves? As I understand it the waves are continuous disturbances in spacetime itself so the wave can never fully dissipate or “miss” me (the observer).

~Max

149

I get that. Any photons emitted while the star was within the bubble would still reach me though. See also the last post.

~Max

150

Yes, so eventually the star would slip over the edge, but for billions of years you would still be getting photons coming from there. But their stream would slow and eventually cease.

151

Okay. So if a binary system is moving away from us, and space is expanding, it will eventually recede at a superluminal rate. Such a system is always propogating gravitational waves, right? As it approaches the edge of the observable universe, said gravitational waves will redshift more and more. Eventually the photons will stop coming so the only measurement or interaction left is the gravitational waves. Will these ever die out, or will they just keep redshifting as t approaches infinity?

~Max

152

I believe gravitational waves propagate at a maximum speed of the speed of light like everything else, so yes

153

Yes, they die out, or yes, they keep redshifting? I’m having trouble understanding how they could die out since, from my point of view as the observer, the system never actually leaves the observable universe.

~Max

154

But it does. If systems didn’t leave the observable universe, since the universe is infinite, we could see every star in the whole universe - which is infinite - and therefore the night sky would be aglow with distant stars.

155

Except no it wouldn’t, because as you explained light dies out.

~Max

156

We are using “event horizon” to mean “the point at which you can no longer interact with them”. What you are describing of time slowing down is the behavior as something approaches the event horizon of a black hole, which is different - that event horizon is due to the black holes gravity while this event horizon is due to receding stars

157

Ok, and so do gravitational waves

Eta: light dying out = going beyond the event horizon

158

From my perspective I never move out of the star’s light cone. As the distance to the star based on measurements available to me approaches the radius of a Hubble sphere, any waves coming from said star are extremely redshifted. Time-dilation follows, such that I will only observe the system approach the edge of the observable universe asymptotically.

Now, I understand that quantization of light into photons means that there is a limit past which no light will arrive. For example, I can assume said system emits x photons in my direction per second (according to the system’s measurement of time). I might observe the system and be able to calculate an upper bound on the number of photons I can expect to receive from that system. After the last photon is received, the light dies out; the system does not (ever) pass the particle horizon. If it was to send a photon one infinitesimal moment before physically moving so far that the intervening space expands faster than a photon can cross it, I would still receive that photon. The only limit is that photons are not sent every infinitesimal moment.

But to my knowledge, gravitational waves aren’t quantized, it’s part of spacetime itself. At every infinitesimal moment a binary system will distort spacetime. It doesn’t come in packets and so it can’t miss me, even if it’s redshifted more and more over time.

~Max

159

Isn’t it the same though, so long as there is redshifting there should be time dilation. With black holes it’s gravitational time dilation, with this (expanding universe) I guess it would just be the Doppler effect.

Bringing the argument full circle…

If it is admitted that gravitational waves will always interact with at least one object - if at least one particle is ever within the light cone of a binary system, and if that system actually leaves the particle’s cosmological event horizon before collapsing - then there will always be meaningful interactions no matter how much time passes. Therefore time has no upper bound, therefore time is infinite.

If time is not infinite because these interactions produce a cycle, the cycle continues forever, and is therefore infinite.

~Max

160

Actually, you do.

Keep in mind that not only is it speeding away from you, but from its perspective, you are speeding away from it. Not only is its radiation, both electromagnetic and gravitational coming from further away and from a more swiftly moving object, you* are also receding from it at an accelerating rate.

Anything it sends at you* not only needs to overcome its own velocity, which at a certain point becomes superluminal, but it also has to catch up with you, which is also superluminal.

The idea that something slows down and becomes redshifted as it approaches a horizon is just an affect of observation in that you can never actually see it cross that horizon, as, if you did, then you would be able to see beyond that horizon, which would defeat the whole point.

The time dilation is only how quickly you observe a clock on the observed particle move, not on how fast the particle itself moves.

It does not actually slow down, it does not actually “hover” right at that most distant edge, it does actually cross it, never to be seen again. It’s not slowly redshifting away as it nears the horizon, it’s been slowly redshifting over the billions of years that its been receding, and as it recedes, not only does the redshift increase, the rate of redhsifting increases as well. The rate of redshifting approaches infinity as it nears the horizon. It doesn’t slowly fade away, it winks out.

Once it has done so, it is no longer casually connected. Nothing that it does will have any affect on you, nothing that you do will have any affect on it. Whether that be gravity or electromagnetic or anything other than tachyons (which [almost certainly] don’t exist), you will be isolated.

This will happen first with galaxies, and is actually happening right now with galaxies, though they are too redshifted to see past the CMB at this time. Then as things break down further, this will eventually happen to everything, every particle, every photon, every gravitational wave (or graviton in case those actually exist.)

If you are a particle, then you will have nothing else in the universe with you, you will be it. Most of the universe at that point will have no particles at all, and that will be an increasing amount as time progresses.

*of course not you, as the only thing that could exist at this point would be photons.

I don’t see why this would be the case.

By this time, there will be no binary systems, there will be no stars or planets or anything at all.

No.

Depends on how you define time, but sure, time is infinite even without your incorrect assertions as to the nature of the cosmological event horizon.

Huh?

Are you talking about the interactions that are not possible as you are no longer causally connected? Or something else.

Roger Penrose would have you believe that once the universe gets to this state, then it is ripe to explode into a new one. Leonard Susskind thinks that’s ridiculous, as that would violate the second law of thermodynamics, and will actually get a bit snippy with you if you bring it up. While I actually respect Susskind more than Penrose on most things, I actually lean towards Penrose on this one.

I personally think that they are both wrong, Susskind for demanding that the universe as a whole, rather than closed systems within it needs to follow thermodynamics, and Penrose for thinking that any information can be carried through to the next cycle. I personally think that it’s pretty simple, in that once you get to this state, you have an evenly expanding universe, an eternally inflating universe. But, you can’t have it be perfectly inflating, and that would violate quantum mechanics uncertainty principle, so you would have small fluctuations. When you have a perfectly expanding universe, small fluctuations become compounding, and grow, until a section of that expanding universe collapses, kicking off a new Era of universal growth. That perfectly smooth inflating universe with just a single fluctuation would be pretty much the definition of the lowest possible entropy state, and all the entropy of the previous universe would have been wiped out and smoothed away by the inflation.

If that were to work out, then that would mean that time is infinite. There would be no beginning, there would be no end. There would certainly be a beginning and an end to a particular collection of matter and energy that the inhabitants consider to be their universe, but the space-time that they are embedded in would be infinite in both temporal and spatial extents.