# Tag Info

### How do we know the universe is expanding, and not that its contents are shrinking?

But even if you see the same # of atoms in that tetrahedron it's only because, ill call it, the universal shrinkrate and it's perception is relative to us and our size just like the speed of light is ...
Accepted

### Where were the first galaxies formed in our observable Universe, at the center or at its outskirts?

If I understand your question correctly (based also on your disagreement with the interpretation of the other answers), what you are asking is whether the first galaxies were formed at the particle ...
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### Where were the first galaxies formed in our observable Universe, at the center or at its outskirts?

The following is a good analogy for the current big bang cosmological model as far as the answer to "where the center of the universe is". Take an ideal balloon,before inflating it, its ...
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### Where were the first galaxies formed in our observable Universe, at the center or at its outskirts?

Our universe does not have a “center.” The distinguishing feature of our location is that we live here. We have excellent evidence that the universe is “isotropic,” which means the time from the Big ...
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### How did neutrinos eliminated from dark matter?

The explanation is technical and has to do with structure formation. The term to look for is "hot dark matter", so-called because hot particles move quickly. In terms of its application, ...
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Accepted

### The expansion of universe releases vacuum energy

It is not certain that dark energy is vacuum energy per se (at least not as a sole contributor), but vacuum energy is in fact inherent to the spacetime as you point out. Since the cosmological ...
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### How is there energy and charge in the universe?

Well, indeed energy is not conserved in curved spacetime. It's only locally conserved. Unless one takes into account the gravitional self energy. Gravitational Pseudo-Tensor of Energy-Momentum One ...
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### How is there energy and charge in the universe?

Experimentally, we see processes like pair production that produce charged particles but conserve charge by producing equal positive and negative charge. We presume that the charged particles we see ...
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### Penrose conformal diagram of Morris-Thorne wormhole

Making a Penrose conformal diagram of a Morris-Thorne wormhole is challenging since the fundamental properties of such a spacetime can't really be represented in two dimensions (there isn't any two ...
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### Do objects outside the Hubble sphere violate special relativity?

Dale's answer is wrong, inasmuch as it suggests that this is related to parallel transport in curved spacetime. If you parallel transport two sublight velocities to the same spacetime location and ...
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### Do objects outside the Hubble sphere violate special relativity?

Yes, this violates special relativity. Special relativity is only valid in a region of spacetime which is small enough that curvature effects are negligible. The Hubble sphere is not a small region of ...
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### What is the theoretical value of this phase space invariant?

I'm going to start by using some relations that appear explicitly in the text you linked. Specifically: $$\mathcal{N} = \frac{g_{s}}{h^{3}} \eta$$ where, for the era of ...
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### Implications of the axis of evil in Big Bang theory and cosmological inflation?

It appears that the results found by Lior Shamir are supportive of the torsion-based cosmological model proposed by Nikodem Poplawski (whose numerous 2010-2021 preprints can be found by his name on ...
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### Calculation of age of Universe

The age of the universe at the age of recombination is calculated to be approximately 370,000 years by using the equation: dt = (1/H_0) (da/a) (1/SQRT(Omega_m/a^3)). This is actually an incorrect ...
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Accepted

### Scalar field equation of motion in FRW metric

You made a mistake when you varied the action. Explicitly, the Lagrangian density is: $$\mathcal L = (-\frac{1}{2}g^{\mu\nu}\partial_\mu\phi\partial_\nu\phi-V(\phi))\sqrt{-g}$$ so the Euler-Lagrange ...
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### How you calculate the age of the observable Universe if the acceleration expansion is not constant?

It is, of course, possible to add dynamical fields to the theory which act as Dark Energy given an appropriate equation of state. In general, different models will indeed lead to a different age of ...
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### How you calculate the age of the observable Universe if the acceleration expansion is not constant?

The time evolution of a universe in which the cosmological constant is actually a variable can indeed be modeled on a computer, and the results compared to observational data. If we imagine a finite-...
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### Understanding the smoothness of Poincaré dodecahedral space

Take the more familiar example of a torus: in identifying opposite sides of a square, the points on the edges have two equivalent positions in the polyhedral complex. The mathematical machinery is ...
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### Calculation of age of Universe

The CMB signal dates from the time of recombination (a misnomer) which was when the temperature had dropped enough to allow stable hydrogen atoms to form and not get immediately re-ionized. This ...
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### Some questions about density perturbations in the early universe

This is not a complete answer; it is a partial answer which justifies the location of the first acoustic peak but not the others. Suppose that at early times (first minutes or so) there is a localized ...
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### Some questions about density perturbations in the early universe

Question 1 The pressure appears because General Relativity generalizes Newtonian gravity to include relativity. Instead of mass sourcing a gravitational field, in GR the source of gravity is the ...
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### If CMB came from the big bang, how come we got to where we are before the CMB arrived?

Welcome to PSE! The main point of confusion seems to be that you think the Big Bang happened at a single location in space. This is a very common misconception, and the name itself probably ...
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### How much new 3d space does the expansion of the universe create?

In our best cosmological model, $\Lambda$CDM, the universe is modelled as a manifold with a metric, $$\text{d}s^2 = - \text{d}t^2 + a^2(t) (\text{d}x^2 + \text{d}y^2 + \text{d}z^2)$$ (I'm neglecting ...
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### How much new 3d space does the expansion of the universe create?

The current radius of the observable universe is estimated to be 14.25 gigaparsecs or 14,250 megaparsecs. Expansion will add (73.24 km/s)*(14,250) = 1,043,670 km/s or 3.38230412e-17 gigaparsec/s to ...
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### Supernova light flash propagating through universe

That Supernova flash of light must contract to its initial size after having crossed the universe.
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### Why is the baryon to photon ratio assumed to be constant?

I have no clear idea why you ask this question, and what information you are looking for. By baryons I am guessing you mean protons and neutrons. I am also assuming you are ignoring dark matter. ...
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### Issues with baryon acoustic oscillations

If you think about the power spectrum of CMB anisotropies, you see various peaks corresponding to different values of l, which represent a particular oscillation mode of the total wave (it is just a ...

### Why is the baryon to photon ratio assumed to be constant?

The baryon to photon ratio is essentially fixed by the annihilation of particles and anti-particles in the early universe. If there had been absolute symmetry between the numbers of particles and anti-...
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### What is the correct gamma factor in FLRW metric in curved spacetime?

In (3.11), $\vec U$ is a 4-vector, and $\vec U\cdot\vec U$ means $\displaystyle \sum_{μ,ν=0}^3 g_{μν}U^μU^ν$, where, in your case, $g = \mathrm{diag}(-1,a^2,a^2,a^2)$. In the FLRW metric as you've ...
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### Right after the Big Bang, how did particles overcome extreme gravity and other forces and manage to fly apart?

Is it being assumed in the above explanations that a dimensional "space" existing as a certain volume of vacuum (i.e., having or containing a complete absence of matter or energy) must have ...
1 vote

### Physical meaning of a time dependent $g_{00}$

Physically this makes no difference. Without this redefinition you just have an arbitrary lapse function $N(t)$ in the $g_{00}$ component, but it is not dynamical. See my answer here Why the FLRW ...
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### If galaxies beyond the cosmological event horizon move faster than light is then that motion a combination of their KE and space expansion?

The motion is primarily due to expansion. The KE is almost always irrelevant. Imagine two objects with equal mass in a common circular orbit about their midpoint M. Assume that M is exactly at the ...
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### Can empty space 'press' galaxies?

Just to deal with these problems physicists introduced the term dark matter. In the first question the expansion rate of galaxy as you said is slowed down by celestial objects' gravity. But Hubble and ...
1 vote
Accepted

### The scale factor of $\Lambda$CDM as a function of time

Dr. phy asked: "can we simply get the scale factor $\rm a$ as a function of time?" For the complete solution containing $\rm \Omega_{r}, \ \Omega_{m}, \ \Omega_{\Lambda}$ simultaneously you ...
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Accepted

### How to get the density parameter of $\Lambda$CDM as a function of time?

Dr. phy wrote: "I’m a little confused how to get the total density parameter Ωtotal=Ωm+Ωr+ΩΛ as a function of time?" First you need the critical density: $$\rm \rho_c=\frac{3 H^2}{8 \pi G}$$...
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### Do the stars in galaxies expand wrt each other because of dark energy?

The mainstream model of cosmology is the Big Bang model, BB,based on the theory of General Relativity. As the words say it, a three dimensional Bang/explosion expands from a center and every part of ...
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