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

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What happened to matter after the BB?

Cosmologists say that before the Big Bang all matter (10^53 Kg) was concentrated in the size of a coin, do you know if (and after how long) all the existing matter was dispersed away from the original ...
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Particle horizon in an empty universe

So in this thread, Can space expand with unlimited speed?, the author Pulsar made amazing diagrams of different horizons and paths for a benchmark model that describes our current universe, and gave a ...
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Cosmos at minimum 250x bigger than our observable Universe, so why then the need for a Big Bang?

Please correct me If I'm wrong but does not the BB only refers to our light speed limited observable Universe (OU) from our home position? Also it is estimated that the Cosmos is minimum 250 times the ...
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Can the Universe reach Equilibrium using Dark Matter and Dark Energy

As I was studying dark energy, dark matter and The Expansion, I thought that if there was enough Dark Matter to stop the Expansion, and enough Dark Energy to make sure it does not end in a big crunch, ...
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Is it possible to determine a moment in time when earth's atmosphere is no longer breathable due to the reduction in atmospheric pressure? [duplicate]

If the universe is expanding, the "pressure" of the "vacuum" in space is decreasing, the earths Atmosphere is expanding, atmospheric pressure is decreasing. Taking into account: ...
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Hypothetically, according to the Standard Model, if the universe were not expanding, could we not cut out the Big Bang and inflation?

Hypothetically, according to the Standard Model, if the universe were not expanding, could we not cut out the Big Bang and inflation and begin the universe in an expanded state and still get the same ...
2 votes
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Total number of particles in the universe (according to Planck Surveyor measures, 2018)

Recently, Marco Ajello et al. (2018) has estimated the total number of photons in the whole observable universe as: $$N_\gamma \approx 4\cdot 10^{84}$$ on the other hand, the ratio of baryons to ...
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Could the passage of time be sinusoidal? [closed]

As we know the universe is moving towards an equilibrium, or high state of entropy. From what I understand, we call this the flow of time. When google announced their time crystal, they call it "...
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Cosmological redshift in static universe [duplicate]

Can cosmological redshift be explained in a static universe?
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How did neutrinos eliminated from dark matter? [duplicate]

I am reading "Dark Matter and Dark Energy" by Brian Clegg. In Chapter 3 it's discussing about cosmic microwave background radiation and the elliptical shape of early universe obtained from ...
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The expansion of universe releases vacuum energy

The quote below has been taken from the Wikipedia article on cosmological constant, https://en.wikipedia.org/wiki/Cosmological_constant. Einstein included the cosmological constant as a term in his ...
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Is Eddington-Lemaître universe relevant to current cosmology?

The "Modern Cosmology" article in Scholarpedia.org mentions the Eddington-Lemaître universe as an example for "an emergence from a static state" as a possible start of the universe....
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Can 1D beings in 1+1D determine if they are in a curved universe?

For hypothetical beings living in a the surface of a sphere, it is possible to determine if their world is Euclidean or curved and closed by making triangles and measuring the sum of the angles (...
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About the determinism of the universe and decidability? [closed]

Note: I have a math/CS background and only basic knowledge of physics. Decidability has to be intended from a logic perspective. To the best of my knowledge, we have no proof if the universe is either ...
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In theory, could any of the theories about the end of the universe leave the universe with no symmetries?

There are several models about the end of the universe (https://en.wikipedia.org/wiki/Ultimate_fate_of_the_universe), the most popular ones are Big Freeze (or Big Chill), Big Rip, Big Crunch and Big ...
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Calculation of age of Universe

The age of the Universe is about 13.8 billion years, measured by light emitted from the time it emerged from opaqueness. But how was the time from the "beginning" to 380,000 years (era of ...
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The values of the universe’s scale factor at the different eras of the universe’s evolution

I’m reading this paper: A unified geometric description of the Universe.... In Fig. (1) they show the different values of the universe’s scale factor at the different eras of the universe’s time ...
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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 &...
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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?...
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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 ...
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3 answers
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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 ...
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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 ...
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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)? ...
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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 ...
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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 ...
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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 ...
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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 ...
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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} ...
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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 ...
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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 ...
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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 ...
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1 answer
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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.
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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 ...
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2 votes
1 answer
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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 ...
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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 ...
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1 answer
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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 ...
2 votes
2 answers
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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 ...
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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'...
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-1 votes
2 answers
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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 ...
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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 ...
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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 ...
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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 ...
1 vote
1 answer
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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 ...
2 votes
1 answer
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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 ...
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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 ...
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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 ...
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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 ...
2 votes
1 answer
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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. ...
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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 ...
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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 ...
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