Questions tagged [universe]

The universe refers to the cosmos; all of space-time and that which exists as part of it. Alternatively, it can refer to the observable universe, which only contains the part we can see. Questions tagged with this should ask about physics at scales the size of the universe or specific properties of the universe

Filter by
Sorted by
Tagged with
0 votes
0 answers
18 views

Are there any theories of the beginning or the end of the universes where all fundamental symmetries would be violated?

There are theories trying to explain the origin of the universe (such as inflationary models) and also theories that try to predict how will the universe end (like the "big freeze", the &...
user avatar
  • 517
0 votes
1 answer
34 views

Oscillating solutions to Friedmann equations

Does homogeneous and isotropic Friedmann cosmologies allow for periodic (simple harmonic solutions) Universes? Can they (universal oscillators) solve the issue of the initial (or future) singularities?...
user avatar
  • 5,373
0 votes
0 answers
45 views

What is the exact ratio of dark energy to mass?

I need to know the exact ratio between mass and dark energy (total dark energy in the universe / total mass in the universe). I could only find it to 2 decimal points (0.68). I need this to make this ...
user avatar
4 votes
3 answers
2k views

Will a light come back within finite years?

In this answer Javier said Imagine the universe was the inside of a ball. We're 3D now, so no one is hiding any dimensions. This ball has a border, except it's not really a border. You should think ...
user avatar
  • 405
2 votes
0 answers
86 views

Could one, in principle, make any predictions using the wavefunction of the universe? [closed]

Do physicists talk about the wavefunction of the universe? What does that wavefunction even mean? Usually, wavefunctions describe probabilities of measurements of a system. But in this case, every ...
user avatar
  • 917
11 votes
3 answers
3k views

What happens when the universe runs out of fuel?

After some X billion years, one would think the stars in the entire universe will run out of hydrogen. What would happen next? Is there any way to get hydrogen out of heavy metals (extreme fission)? ...
user avatar
  • 721
1 vote
0 answers
51 views

How do you define present from physics point of view [closed]

So how would you exactly define present, aside from fact that time is an illusion; is it the age of the universe in this sector that we experience subjectively as present, does it have to do anything ...
user avatar
  • 11
1 vote
1 answer
31 views

Understanding primordial Statistical Fluctuations in the Universe

I am trying to answer the question, Whether gravitational collapse began from Statistical fluctuations in the early Universe? In order to answer this I must find out the total number of particles in ...
user avatar
0 votes
2 answers
71 views

What is there at a point the universe hasn't expanded past yet? [duplicate]

(Please don't mark as a duplicate) If the universe is constantly expanding that means that there is a point the univese hasn't expanded past, with that what would be past that point? This isn't about ...
user avatar
-1 votes
0 answers
45 views

What is at the edge of space? [duplicate]

if the universe is constantly expanding that means that there is an end where the universe hasn't expanded yet if you could be at that before it was to expand more, or go past/into that edge, what ...
user avatar
0 votes
0 answers
38 views

Friedmann equation solution and age of the universe in terms of $H_0$ and $Ω_{(r,0)}$ for an open/closed universe [duplicate]

I cam currently doing a problem involving Friedmann equation for $a(t)$ and solving for $t$ when $a(t)=a_0$ in order to get the age of the universe. Since the integrals are quite nasty, I want to make ...
user avatar
  • 3
3 votes
1 answer
83 views

Deriving the age of the universe

I am trying to work out the solution to exercise 8.4 from An Introduction to Modern Cosmology by Andrew Liddle. I could derive the Friedmann equation as below, $$\dot{a}^2 = H_0^2 \left[\Omega_0a^{-1} ...
user avatar
0 votes
0 answers
34 views

Is the set of reachable arrangements of an indeterministic universe (of a given material substrate) sensitive to its initial arrangement?

Suppose two equally massive universes have an identical material substrate of the same fundamental particles. Suppose then that these two universes initially have different arrangements of these same ...
user avatar
0 votes
1 answer
53 views

Is the set of reachable states of an indeterministic universe sensitive to its initial conditions?

Suppose two universes with the same amount of mass-energy and evolving according to the same natural laws, but having different initial conditions. Is the set of states that are reachable by the ...
user avatar
0 votes
0 answers
19 views

Dark matter and un-smoothness in spacetime [duplicate]

Since dark matter currently is only observable with its gravitational effects and nothing else can we theorize that dark matter is only non-smoothness in spacetime that has been there from the Big ...
user avatar
0 votes
1 answer
55 views

If I moved in a straight line forever, would I hit something?

Say you could move infinitely fast/gravity doesn't affect you. You move in a straight line. Do you hit anything, like stars or black holes? Not talking about cosmic dust.
user avatar
2 votes
0 answers
72 views

Could the universe be a 4-ball?

I recently thought of the idea that the universe could be an infinite 4-ball. The Big Bang would be its centre, and time would be outward from its centre (one layer would be one point in time). I ...
user avatar
2 votes
1 answer
66 views

About a hypothetical universe

Imagine a universe dominated by matter, but it is balanced with a cosmological constant $\Lambda=4\pi G\rho$ so the universe is static ($H=0$). However, what would happen if some of that matter turns ...
user avatar
-2 votes
0 answers
9 views

Calculation of positive energy of the universe [duplicate]

Being new to this site, I find it very difficult to navigate. I just had a question deleted for reasons I don't understand. So, I will try again. Two part question--1. How is the positive energy of ...
user avatar
0 votes
0 answers
24 views

Total energy of the universe calculation [duplicate]

If the total energy of the universe is zero, how is the positive energy calculated--is it mass energy, kinetic energy or both? Also if dark energy is causing the universe to expand, how can the total ...
user avatar
0 votes
1 answer
69 views

Are there any mathematically conceivable arrangements of an indeterministic universe that are not physically possible?

Are there any mathematically conceivable arrangements of this universe (particularly ones that respect its mass-energy content) that cannot in principle be achieved through its evolution in accordance ...
user avatar
2 votes
2 answers
77 views

Space-time continuum expansion

I still don't understand how the expansion of the universe works. If the universe is made up of an infinite number of points that make up space-time, then how can space expand or stretch. Common sense ...
user avatar
  • 161
0 votes
1 answer
34 views

Theory for the relative scale of objects?

I've been pondering what Prof. Brian Cox said in an interview (still searching for it) where he said something along the lines of: we (human beings) are very small relative to the earth, universe. I'...
user avatar
  • 101
-1 votes
2 answers
109 views

Can space only be infinite? [closed]

I have read before that if you could just go fast enough, as a thought experiment, and you move in a straight line, in any direction, that you eventually might reach the spot from which you started. I ...
user avatar
  • 11
1 vote
0 answers
24 views

Continuity equation for the estimation of cosmological dimensionless parameters

I want to set up a system of equations to find the dimensionless density parameters $\Omega_i$ as a function of $N=\ln(a)$, from the continuity equation: $$\dot{\rho_i}+3H(p_i+\rho_i)=0$$ where the ...
user avatar
1 vote
0 answers
43 views

Are there any Big Bounce models where the laws of physics (even the most fundamental ones) would change between cycles?

Big Crunch-Big Bang models (usually called Big Bounce models or cosmological oscillatory models) are highly unlikely to happen due to the discovery of the accelerated expansion of the universe. But ...
user avatar
  • 517
1 vote
3 answers
113 views

Is the solar system sitting in the centre of curved spacetime and if so, are we viewing the rest of the universe from inside that "bubble"?

I read an article about a huge bubble being discovered in which the solar system sits bang in the middle. It got me thinking about the curvature of spacetime. The bubble was created by several ...
user avatar
1 vote
1 answer
63 views

How far back in time can we say $\rho_{\gamma}(t) = \rho_{\gamma, 0}a(t)^{-4}$?

The equation for the density of light in the universe is $$ \rho_{\gamma}(a) = \rho_{\gamma, 0}a^{-4} $$ where $\rho_{\gamma, 0}$ is the density of the CMB today. I understand why this scaling ...
user avatar
1 vote
1 answer
68 views

Can there be negative energy within the classical general relativity?

The positive energy theorem (Witten and Yau were awarded the Fields medal in mathematics in part for their work on this topic.) says that "the gravitational energy of an isolated system is ...
user avatar
-1 votes
2 answers
58 views

Could somehow the fundamental laws and symmetries of physics change or be broken? [closed]

There are some theoretical processes (like vacuum decay in quantum field theory) that could change the physical constants of the universe. Similarly, in inflation theory, various models predict that ...
user avatar
  • 517
1 vote
2 answers
45 views

Assuming that heat death or Big Freeze would be the final outcome for the universe, what would happen with energy?

According to Big Freeze, the universe will expand forever and it will reach a state of thermodynamical equilibrium. It is said that the temperature of the universe will reach an asymptotically ...
user avatar
  • 517
3 votes
1 answer
93 views

Does whether or not inflation occurred change the estimated age of the universe?

When we say "age of the universe," I assume this means the time interval for the Euclidean FLRW metric $$ ds^{2} = -dt^{2} + a^{2}(t)\Big(dx^{2}+dy^{2}+dz^{2}\Big) $$ from when the scale ...
user avatar
2 votes
1 answer
57 views

Reversal of the psychological arrow of time on the contraction of the universe [closed]

In Brief History of Time, it says that the people living in a contracting phase (although unsuitable for living or even existence itself, but lets assume) would remember the future but not the past. ...
user avatar
  • 130
1 vote
0 answers
18 views

What are the appropriate densities and temperatures of constituents of the universe after reheating?

In slow roll inflation, the inflaton field has to decay into the currently observed constituents of the universe at the end of inflation. This process is called reheating. What are the experimental ...
user avatar
  • 1,449
0 votes
1 answer
62 views

How to calculate the redshift of reionization?

I am trying to calculate equalities given Omega parameters. For example given, $\Omega_L = 0.6889083, \Omega_M = 0.311, \Omega_R = 9.17$ x $10^{-5}$ and $\Omega_K = 0$ $H_0 = 67.7$ km/sec/Mpc The ...
user avatar
  • 113
2 votes
1 answer
56 views

Could the metric of the universe have the following form?

If the universe's metric had the following form $$ds^{2}=N(r,\tau)c^{2}d\tau^{2}-a^{2}(\tau)\big(\frac{1}{1-kr^{2}}dr^{2}+r^{2}d\Omega^{2}\big)$$ for $N(r,\tau)$, $N(\tau)$ or $N(r) \neq 1$. Would ...
user avatar
1 vote
3 answers
84 views

Energy conservation and time uniformity

Reading Peter Atkin's book Four laws that drive the universe, I cannot see the link between time uniformity and conservation of energy the way it is tackled in this excerpt: " Noether’s theorem, ...
user avatar
  • 111
2 votes
3 answers
134 views

Why is the ratio of dark matter to normal matter larger in galaxies than the cosmic average?

There seems to be a discrepancy between the ratio of dark matter to normal matter in the Universe (about 5 to 1 according to $\Lambda$-CDM) and the ratio of the average dark matter halo mass to the ...
user avatar
  • 63
0 votes
1 answer
66 views

Thought experiment with the speed of light

Every time we look at a star we are looking to the past. That's because the light of that star needed to travel long distances at the speed of light. When the light reaches our sight maybe the star no ...
user avatar
  • 103
3 votes
1 answer
49 views

Which of these 2 ideas is correct about a "bubble" of false vacuum decay?

The whole volume of the bubble is true vacuum. Only the surface layer of the bubble is true vacuum while the interior has turned back to false vacuum.
user avatar
3 votes
2 answers
241 views

Knowledge of the neutrino density in the Universe

If detecting neutrinos is so difficult, how do we have an estimate of how many of them pass through a cubic centimeter per second?
user avatar
  • 335
2 votes
2 answers
141 views

Space is flat but spacetime is curved?

In the picture below a triangle has been drawn on a spherical shaped object. The angles add up to 67+48+73=188 degrees. Since the surface of earth is also almost spherical therefore an experiment can ...
user avatar
  • 655
-2 votes
1 answer
62 views

IF heat was the absence of cold, how would that change physics [closed]

Back in high school, I had an argument with my physical science teacher. She said cold is the absence of heat. I asked why it couldn't be the other way around. She simply stated that was not possible. ...
user avatar
0 votes
3 answers
125 views

Is there evidence for the existence of time? And what's eternalism (block universe)? [closed]

I have been told that: "time is an illusion. Rather than a chronological progression of events, all events occur simultaneously in space, and so right now, dinosaurs exist, WW2 just ended, you're ...
user avatar
  • 133
1 vote
2 answers
75 views

Could we estimate the total energy of the universe?

So I know that we do not know the sum of all energy in the universe, but why can we not just estimate with the following logic? (That I assume has some fatal flaw preventing anyone from guessing the ...
user avatar
  • 107
17 votes
1 answer
333 views

Did Einstein really invent the cosmological constant to make the universe static in his 1917 paper?

The popular account of Einstein inventing the cosmological constant goes like this: Einstein finds that the Einstein Field Equations predict an expanding universe Unable to accept this, Einstein adds ...
user avatar
0 votes
3 answers
108 views

What state of matter is the entire known universe?

What state of matter is the entire known universe? I know it contains all forms, but as it expands into the infinite nothingness, what state of matter does it act as? I know little about physics, but ...
user avatar
0 votes
2 answers
120 views

Is most of the energy in the universe potential energy?

So I asked a question about what would happen in regards to gravitational potential if I left earth and then vaporized it. The answer I got was that the Mass would still remain the same and even if ...
user avatar
  • 107
-5 votes
2 answers
72 views

Is time continuous or discontinuous? [duplicate]

As I notice I sometimes feel as if time is discontinuous it’s like a comic book where each act is planned out and the main character just comes there. Please give an explanation and correct me if I am ...
user avatar
  • 21
0 votes
1 answer
55 views

In theory how much bigger would the universe be if Inflation had lasted longer?

In theory, Inflation started at $10^{-36}$ seconds after the Big Bang and ended at $10^{-32}$ seconds. But let's imagine Inflation had lasted one second instead. In theory how much bigger would that ...
user avatar

1
2 3 4 5
32