# Tag Info

20

TL; DR Somewhere between now and a few hundred billion years time. (For a co-moving volume) Now read on. If stellar remnants are included, then the answer is very far in the future indeed, if and when the constituents of baryons begin to decay. So let's assume that "stars" means those things that are undergoing nuclear fusion reactions to power their ...

16

It is a common misconception that galaxies receding faster than light cannot be observed. There are two versions of this misconception: Galaxies that are now receding faster than light cannot be seen. If we observe a galaxy today, it may recede faster than light now, but when it emitted the light we see now, it did not. Both are incorrect. Intuitive ...

9

Your 3 points are spot on. There is a finite number of galaxies we could theoretically reach for the reasons you say. The further away the distant galaxy the greater the expansion of space between us and that galaxy and beyond a certain distance, galaxies can't be reached, even by the speed of light. We can see galaxies that we couldn't possibly travel ...

8

Aside from being, I suspect, totally incompatible with GR, the model fails to match observations. The universe appears to be homogeneous and isotropic. In the model you suggest, Hubble's law would be different depending on which direction you were looking. In particular, looking away from the black hole, galaxy redshifts would increase to some asymptotic ...

8

The answer here is very similar to if you were asking about light. In principle gravitational waves might allow us to fractions of a second after the big bang. Electromagnetic waves can see back to where the cosmic background radiation formed, about 400,000 years after the big bang. You are right, the universe has expanded. At the present epoch it is ...

6

The main obstacle with the Oscillating Universe model is the pesky second law of thermodynamics — that entropy always increases within an isolated system, and never the reverse. Thus, as time progresses, entropy increases, until it (as far as we know) reaches its maximum — the heat death or Big Rip. Consider the Big Bang, when entropy was at its minimum, or ...

6

However, due to the Law of Conservation of Energy, I suspect that energy will be all that is left, only in equilibrium meaning no ability for energy to form matter. The law of conservation of energy states that energy is conserved in a closed system. This means the Universe, which is likely an open system, does not necessarily have to obey that conservation ...

5

The amount of matter in the universe is directly related to the curvature of space itself. We can look at the Friedmann Equations to see how this works: $$H(t)^2 = \frac{R'(t)^2}{R(t)^2} = \frac{8\pi G}{3}(\rho_{m} + \rho_{r}) + \frac{1}{3}\Lambda - \frac{c^2}{R^2\mathscr{R}^2}$$ This is the equation for determining the scale factor for finding distances ...

5

Yes, over time the accelerating expansion of the universe (assuming the $\Lambda$CDM model is right) will separate gravitationally bound clusters of galaxies from each other with exponentially growing distances. However, this will not happen in 15 billion years. This excellent paper analyses the expansion in detail. What takes about 17 billion years is for ...

5

A finite universe is said to have a "closed geometry", or to be "positively curved", meaning that, in principle, you may travel in a straight line and eventually return back to your starting point. In the 2D analogy, the surface of Earth is positively curved, and if you travel 40,000 km straight, you're back where you started. A finite universe that does ...

4

There are several theories on how the universe might end by time stopping. The problem with cosmology and some other parts of theoretical physics is that it cannot be proven, nor reproduced or tested. This makes it often more a faith/religion than a science. So, if you notice some irony in my answer, please, it's because of this. There are two types of ...

4

The Big Rip happens if the equation of state for the dark energy has $p/\rho = w<-1$, and all empirical data give us $w\approx -1$. A Big Crunch requires a pretty high value of $w$ (it must go above -1/3 to just stop the acceleration), but of course if dark energy is changing over time it might do that. So tentatively ruling out Big Crunches is the ...

3

Yes. Quoting values from wikipedia (which in turn cites Ade et.al. in Astronomy and Astrophyics 517), the contribution of matter (both Dark and visible matter) is $$Ω_\text{mass} ≈ 0.315±0.018$$ The contribution of photons and neutrinos is small, and within the boundaries of error of the other terms: $$Ω_\text{relativistic} ≈ 9.24×10^{−5}$$ And the ...

3

Your scheme of creating energy from the tidal forces in a pair of orbiting planets cannot work because of the law of conservation of energy (aka the 1st law of thermodynamics), which states that energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another. It takes energy to set the 2 planets in ...

3

The standard picture of vacuum decay events is very much like nucleation in a phase transition: it starts somewhere, and then expands at a constant speed - lightspeed in this case. So the future lightcone of the nucleation event will be true vacuum. In an accelerating expanding universe like our current $\Lambda$CDM cosmology future lightcones actually ...

3

You are thinking about gravity and how it affects the universe as a whole in the wrong way. The dynamics of the universe are governed by the Friedmann equations, which can be solved to say how the scale factor of a homogeneous, isotropic universe behaves with time. If there is enough gravitating matter/energy density in the universe then the scale factor ...

3

Heat death of the Universe doesn't mean that there is no energy in it, it means it has reached thermodynamic equilibrium and hence no useful energy in it. This could in theory occur in an infinite Universe. Of course thermodynamics is statistical in nature, so an argument could be made that some extremely unlikely event in a region of the Universe could ...

2

I've found the answer to this is actually very interesting and (to me, at least) surprising — it is not just the light cone that shrinks, but the cosmic event horizon as well! To explain this, let us first look at a de Sitter universe. The scale factor $a$ of a de Sitter universe can be expressed as a function of time with: $$a(t) = e^{kt}$$ where $k$ is ...

2

No one can say for sure because, while we have theories which fit our observations quite well, those theories require that the universe be considerably bigger than the part we can see -- light simply not having had enough time to get to us yet. But since we don't have observations of this extended universe, we run a significant risk that we're wrong in some ...

2

The current model ($\Lambda$CDM) predicts that space will expand exponentially, and keep on doing it indefinitely far into the future. The end result, after the stars burn out, galaxies disperse, protons decay and black holes evaporate is a very thin soup of stable particles (see Adams & Laughlin 1997 or their popular book The Five Ages of the Universe). ...

1

Yes ... and no. Experimentally, there's no indication that the laws of physics are changing (c.f. conservation of energy: if the laws of physics change with time, energy would not be conserved via Noether's theorem). So if you're an empiricist, you'd conclude that the universe is stable (or at least its mean lifetime must be more than ~14 billion years, the ...

1

It will always expand (probably). The mass and energy in the universe curves space, and it seems that there is exactly the right amount of mass and energy to keep the universe expanding forever. Had you asked this question 20 years ago the answer would have been "we don't know". Recent discoveries of "Dark Energy" suggest that the expansion of the ...

1

You are not doing anything wrong. Positively curved empty universes are forbidden. Since as you figure it out the scale factor would be imaginary.

1

I see that you are using the convention that a has units of distance and is like the radius of curvature of the universe, and you are getting that either a or t would need to be imaginary. So you can interpret that in two ways, either you say that is impossible, and conclude that it is impossible for an empty universe to be closed, or you attribute some ...

1

So in this model, all of the observable universe is expanding. It is simply that it is part of a larger universe which is being locally stretched, as part of a an accelerating convergence to a central point. I think the answer is that we cannot distinguish this model from an expanding universe (assuming the structure of the convergence is very carefully ...

1

For a contracting universe, space would contract as a ratio, and hence even if we could only see a small distance, we would still see blue-shifted galaxies due to the fact that the galaxies further from the hypothetical centre of the universe (see here for why I say hypothetical) would be moving faster than those closer, and hence any two points in space ...

1

This will take about 2 trillion years. Given everything we know now, it's not too hard to predict the eventual fate of the universe. Of course there may be some changes as our knowledge advances, but I think the general course of events will occur as we expect them to. Based on our current knowledge, if you run through the math, you find that after about 2 ...

1

In less mathy words, the Big Crunch scenario occurs if the ratio of the total density of the Universe to its expansion rate is sufficiently large$^\dagger$. As I understand your question, you're basically asking, "Why would a sufficiently dense universe not simply contract into a clump? Why does it have to pull space itself with it?" And you basically ...

1

For a more analytical and much much less philosophical version of an answer than MacUserT's, here it is. According to MacUserT, Now, what has this to do with the ending of the universe in respect to time? The theory you mentioned as heat death is such a transcendental theory. The theory states that the universe expands until the universe is too ...

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