When will the Universe end?

[Jimmy87]

Hi,

I was watching a documentary by Professor Brian Cox and he said human life is only possible for 10^-86% of the life of the universe. He based this on the fact that the age of starlight we are currently in is a very small event in the life span of the universe and human life obviously can’t exist without a star. He said the universe will end when the last black hole evaporates. I have put a short YouTube clip showing that part of the documentary.





Not very good quality but everything I am talking about he says at 44mins to 46mins

I was quite amazed by this statement as we are in this tiny window of opportunity but then my bubble was burst when I did some researching. Some models using multiverse models with cut-off parameters predict the universe may only last as long as our own Sun - 4 billion years! Some even 2 billion!

So I just wanted to ask people’s views on the life span of the universe. How can Brian Cox make such sweeping statements when the universe may only last another 4 billion years?

Thanks for any insights

 

[kurros]

Well it's a bit of a hyperbolic thing to say, and a bit of an arbitrary definition of "end". There is no actual end to the universe in that scenario, its just that nothing much is happening once the last black holes evaporate. There will still be cold dead stars and planets floating about, since I don't think there is any reason why black holes would swallow everything eventually. Although maybe they will get swallowed, I guess the orbits of everything will slowly decay by gravitational wave emission. Well anyway the universe won't end in any literal sense, the universe will just become very boring eventually.

 

[Jimmy87]

Well it's a bit of a hyperbolic thing to say, and a bit of an arbitrary definition of "end". There is no actual end to the universe in that scenario, its just that nothing much is happening once the last black holes evaporate. There will still be cold dead stars and planets floating about, since I don't think there is any reason why black holes would swallow everything eventually. Although maybe they will get swallowed, I guess the orbits of everything will slowly decay by gravitational wave emission. Well anyway the universe won't end in any literal sense, the universe will just become very boring eventually.

Thanks. Since 10^-87 is such a small percentage is it misleading to emphasise how priveledged it is to be in the current window for human life to exist. For example, would it be fair to use that and since there are about 10⁸⁶ atoms in the universe, that if you lined up all the atoms in the universe then only 1 atom out of all those atoms is the same as the odds of being in the window of the universe we are currently in? So it is like the universe blinks and all possible life forms have been and gone? Is the window we are in that special?

Are the cut-off theories using the multiverse idea any good? Like this one -

https://www.google.co.uk/amp/s/io9....rs-so-that-the-laws-of-physics-make-sense/amp

Could the universe really only have 4 billion years left? It seems ridiculous that using two conflicting theories means that life could exist for 100% of the universe’s life vs 10^-86%.

 

[JMz]

One thing that multiverse theories bring into this discussion is the centrality of selection bias: We are in one of the universes that has life, not one of the (seemingly enormously more numerous) ones that doesn't have the right fundamental constants to allow that.

For a single universe, you could make the case that the same selection bias is present: The only epochs in which people can think about the issue are epochs in which people can exist. For example, it seems that the pre-inflation universe may have been too simple to allow complex organizations to exist (especially, ones that can metabolize and reproduce, to allow natural selection). Of course, our understanding of the pre-inflation universe is as uncertain and model-dependent as our understanding of the far future of the universe.

 

[Grinkle]

Could the universe really only have 4 billion years left?

For me the below is a completely un-parsable translation of whatever Bousso's work is. I have no idea what the below snip from the referenced article means.

"According to UC Berkeley physicist Raphael Bousso and his team, the statistical cut-off introduced by physicists to get manageable probabilities behaves like an actual structure of the multiverse. So what does that mean? Basically, it means that the universe must end for the laws of physics to have any meaning, and since the statistical cut-off to the size of space-time already allows us to get the right values for our universe, then that cut-off might actually be a very real boundary to the universe, beyond which further expansion is impossible."

And the summary -

"Now, what does it mean for the universe to end? Whatever the ending is, it will take the form of a physical catastrophe, but it's unlikely we will ever see it coming. We shouldn't be able to see the catastrophe happen elsewhere, as that means we would be causally ahead of it, which the physicists say is unlikely. More likely, the entire thing will unfold at the speed of light (or, maybe, in some weird way, simultaneously across the entire universe), meaning our descendants will never have to watch the universe burn."

is just word salad to me.

@JMz makes a very good point about the anthropic principle explaining our 'luck'. Fish are not particularly lucky to be born in water instead of on dry land, no matter the ratio of water to dry land in whatever area they exist. Its the only place they can end up.

 

[platosuniverse]

The fact is, Brian Cox is a good Physicist but he's also a pop scientist. So he sensationalize things to sell books or get more views on his documentaries. There's been a few things I have heard from Cox that I disagree with.

The truth is, we don't know. This is because we don't know what dark energy is. Some people don't think dark energy exists. We can plot out the evolution of the universe with accuracy because of the cosmological constant. We don't know the nature of dark energy so we don't know if expansion will slow in the future or if there's some mechanism we don't know about that causes some sort of big crunch. Look at this graph.

Right now, the universe is dominated by dark energy so we're on a runaway expansion. If there's nothing to stop this expansion then I don't see how it stops. Here's some calculations.

When will the Universe reach 100 billion light years in diameter? When it's 14.9 billion years old, just 1.1 billion years from now. At that point, the Universe will be 73% dark energy and the expansion rate will have dropped to 65 km/s/Mpc. Not much of a change. But as we go forward in large steps, the changes get very dramatic.

By time the Universe is 24.5 billion years old, a little more than 10 billion years in the future, it will be 94% dark energy, the expansion rate will be 57 km/s/Mpc, but the observable Universe will be 200 billion light years in diameter.

At an age of 37.6 billion years, the Universe will be 99.4% dark energy, the expansion rate will be 55.4 km/s/Mpc, and now the Universe will be 400 billion light years in diameter.

And now, every 12.2 billion years after that, the size of the Universe will double, with the expansion rate leveling off at 55.4 km/s/Mpc. This means the Universe will hit 1 trillion light years in diameter when it's 54 billion years old; 10 trillion light years at 86 billion years; 100 trillion light years at 118 billion years; and a quadrillion light years in diameter at 149 billion years. By time the Universe is ten times its current age, it will be nearly ten thousand times its current size. That's the power of exponential expansion.

https://www.forbes.com/sites/starts...n-how-big-will-the-universe-get/#1c4df3a71f52

Again, these numbers only mean something if everything stays the same and none of the other theories are correct. So like I said, we don't know.

 

[phinds]

How can Brian Cox make such sweeping statements ... ?

Because he is writing science popularization, not science. That is, it's entertainment, not science.

 

[Jimmy87]

Because he is writing science popularization, not science. That is, it's entertainment, not science.

Thanks to all. Physics forums is such a wealth of information. I think my bubble has definitely burst. How amazing it would be if the window we were in was that small.

Again, thank you for such insightful responses. Physics forums is unparalleled with its users and community!

 

[rootone]

... How amazing it would be if the window we were in was that small.

It's not that small, and the bits of the Universe where life cannot exist at all are not very interesting even if they are most of it,

 

[Jimmy87]

It's not that small, and the bits of the Universe where life cannot exist at all are not very interesting even if they are most of it,

10^-87 is pretty damn small. Like I said, if that’s true, the universe blinks and all possible life forms have been and gone.

 

[rootone]

I could exist on Earth a million years ago, and I probably could exist in a million years in the future on Earth.
Given that my lifetime is at best 100 years, I am not bothered that I don't get to see the entire Universe

 

[JMz]

Because he is writing science popularization, not science. That is, it's entertainment, not science.

Not really. It's because he is writing science popularization, period. That doesn't make it not-science, even if entertainment is an important component. (Would you say the same of Cosmos, in either version? I would not.) Conversely, he is making unsupportable statements, then that's not science popularization, it's only entertainment: He doesn't get a free ride on his facts just by being a scientist in his other life.

 

[phinds]

Not really. It's because he is writing science popularization, period. That doesn't make it not-science, even if entertainment is an important component. (Would you say the same of Cosmos, in either version? I would not.) Conversely, he is making unsupportable statements, then that's not science popularization, it's only entertainment: He doesn't get a free ride on his facts just by being a scientist in his other life.

Well, pop-sci presentations always get SO much wrong that I consider them all to be just entertainment, not education. They're entertaining to watch and read and I do a lot of both but I know better than to take them seriously.

 

[JMz]

Well, pop-sci presentations always get SO much wrong that I consider them all to be just entertainment, not education. They're entertaining to watch and read and I do a lot of both but I know better than to take them seriously.

Perhaps a matter of taste, then. For the best -- and here I would include Asimov's articles, for instance, as well as, say, Cosmos -- I can enjoy the presentation, even if the facts are completely familiar: "Gee, that was a really good way to explain it", or "That's a lot clearer than the convoluted way I usually think about it", or "That was a memorable visual that really makes the point".

Of course, all of that does presume that the facts are not being distorted in the interest of entertaining visuals or wordplay, which is not always the case.

 

[kurros]

10^-87 is pretty damn small. Like I said, if that’s true, the universe blinks and all possible life forms have been and gone.

It doesn't matter how small it is, it isn't surprising that we exist during the period of time that the universe supports life. Same as how it isn't surprising that we exist on a planet that supports life, despite the great majority of planets (and the rest of space) being inhospitable to life. It would be impossible to observe a different result.

 

[JMz]

It doesn't matter how small it is, it isn't surprising that we exist during the period of time that the universe supports life. Same as how it isn't surprising that we exist on a planet that supports life, despite the great majority of planets (and the rest of space) being inhospitable to life. It would be impossible to observe a different result.

It is, of course, an interesting question whether we (technological life in general) can influence the result, perhaps by creating new universes with constants of nature that favor complexity, life, and "reproduction" of such favorable universes -- and whether ours is even the result of such a technological construction. That would not, however, change anything about the fate of own universe, the question of the OP.

 

[kimbyd]

Really good resource for this:
https://en.m.wikipedia.org/wiki/Future_of_an_expanding_universe

 

[JMz]

Really good resource for this:
https://en.m.wikipedia.org/wiki/Future_of_an_expanding_universe

Nice. (I fixed a typo, too. ;-)

 

[newjerseyrunner]

I disagree with the concept that the universe is done once the black holes evaporate. There is still stuff moving around the universe and interacting at that point. To me, the end of the universe would be when the universe has expanded and cooled to the point that there is a single photon per Hubble volume. By definition, nothing else could ever happen since there is nothing to interact with.

 

[JMz]

As I recall, the single photon-per-Hubble volume epoch starts before the big BHs evaporate. So they go out in a blaze of glory that is seen by no one, and then you wait a "little" longer for those final photons & leptons to disperse. No?

 

[Simon Peach]

If you are relying on Hawking Evaporation, that's going to be a very very long time.

 

[JMz]

Quite so. :-)

 

[Aufbauwerk 2045]

Discussions about the "beginning" or the "end" of the "universe" make no sense to me, because I don't understand the terms I have put in quotes.

 

[JMz]

Discussions about the "beginning" or the "end" of the "universe" make no sense to me, because I don't understand the terms I have put in quotes.

Within PF, the first and last have a variety of meanings that are generally well understood in context, including in this thread, and post #2 already touches on the middle one.

 

[Dr_Zinj]

The fact is, Brian Cox is a good Physicist but he's also a pop scientist. So he sensationalize things to sell books or get more views on his documentaries. There's been a few things I have heard from Cox that I disagree with.

The truth is, we don't know. This is because we don't know what dark energy is. Some people don't think dark energy exists. We can plot out the evolution of the universe with accuracy because of the cosmological constant. We don't know the nature of dark energy so we don't know if expansion will slow in the future or if there's some mechanism we don't know about that causes some sort of big crunch. Look at this graph.

View attachment 225944

Right now, the universe is dominated by dark energy so we're on a runaway expansion. If there's nothing to stop this expansion then I don't see how it stops. Here's some calculations.



https://www.forbes.com/sites/starts...n-how-big-will-the-universe-get/#1c4df3a71f52

Again, these numbers only mean something if everything stays the same and none of the other theories are correct. So like I said, we don't know.

FWIW, anytime I see a hockey stick graph with us at the beginning of the up curve, I become ultra skeptical of any claims about behavior into the future.

 

[JMz]

FWIW, anytime I see a hockey stick graph with us at the beginning of the up curve, I become ultra skeptical of any claims about behavior into the future.

Presumably that's because reality is an "open" system, in which new phenomena can come along unexpectedly to interrupt wild increases, in practically any domain.

However, cosmology is different, because gravity is different. People had the same skepticism about singularities when black holes were first predicted, and for the same reason. But black holes are real.

Will anything, ever, interrupt that infinite expansion? Quite possibly, but the best bet is that it will not happen before an enormous increase, at least. In fact, that already happened once before. This isn't naive curve-fitting and spreadsheet projections.

 

[kimbyd]

FWIW, anytime I see a hockey stick graph with us at the beginning of the up curve, I become ultra skeptical of any claims about behavior into the future.

That's a very poor line of reasoning.

First, the expansion of our universe isn't very close to a "hockey stick" graph: the accelerated expansion has been going on for roughly half of the current age of our universe. You can make it look like that if you decide to use a logarithmic time axis. The logarithmic time axis can be useful in highlighting the behavior in the early universe, where multiple events happened on much shorter timescales than occur today. But it hides just how long the accelerated expansion has been going on.

Second, the real way to get into whether or not the accelerated expansion is likely to continue is to examine the various models which explain the accelerated expansion. The simplest of those models, the one with the fewest assumptions, is the cosmological constant, which will result in the accelerated expansion continuing forever. In fact, you'd be hard-pressed to find an alternative model which explains current data and doesn't predict the same future path of expansion. You have to go to exotic possibilities such as quantum vacuum decay to get away from the accelerated expansion.

Finally, the "hockey stick" is usually terminology applies to global warming. If you think that graph is worthy of skepticism, you really need to learn more about the science surrounding global warming. Not in this thread, though (and possibly not on this forum, because there is so much junk science claiming that global warming is a hoax).

 

[PeterDonis]

If you think that graph is worthy of skepticism, you really need to learn more about the science surrounding global warming. Not in this thread, though

And any such discussion must be within the PF guidelines for this particular topic, which is known to be a contentious one:

https://www.physicsforums.com/threads/climate-change-global-warming-policy.757267/

 

[JMz]

Finally, the "hockey stick" is usually terminology applies to global warming. If you think that graph is worthy of skepticism, you really need to learn more about the science surrounding global warming. Not in this thread, though (and possibly not on this forum, because there is so much junk science claiming that global warming is a hoax).

Actually, the terminology has been applied frequently to investment outcomes, especially for technology startups, and probably for longer than to global warming. (That's why I mentioned the subject, infamous in startup circles, of "spreadsheet projections".)

I can't speak to the background of usage here. But I would not presume that it came from the irresponsible, and politically motivated, junk science of global-warming denial.