Questions tagged [cosmology]

The study of the large-scale structure, history, and future of the universe. Cosmology is about asking and answering questions about the "big picture" - the extent, origin, and fate of everything we know.

773 questions with no upvoted or accepted answers
Filter by
Sorted by
Tagged with
4
votes
0answers
165 views

Temperature of the CMB when the Earth formed and the faint young Sun paradox

The cosmic microwave background (CMB) has a modern temperature of about 2.7 K. At the time of the origin of the CMB, about 13.6 billion years ago, it had a temperature of about 3000 K. ...
4
votes
4answers
247 views

How do we know the universe is expanding and wasn't already infinite?

From what I know, the decision that the universe was expanding came from the discovery of the redshift and that other celestial objects, such as galaxies, were moving away from us; the farther they ...
4
votes
2answers
89 views

Physical differences between our universe and strongly negatively curved one

Interestingly, the idea of a negatively curved universe isn't entirely science fiction. In particular if decrease a single physical constant (known as the density parameter) to be less than one, the ...
4
votes
2answers
890 views

Attractive gravity has negative energy, what about repulsive gravity in the inflation phase?

Alan Guth's cosmic inflation theory posits that about 10^-38 second before the Big Bang which led to our particular universe, a tiny patch of space doubled in size more than 100 times, from sub-atomic ...
4
votes
1answer
240 views

Why is Newtonian cosmology correct for curved space?

The Newtonian model of an expanding Universe gives Friedmann's equation exactly for non-zero spatial curvature $k$ (see http://hyperphysics.phy-astr.gsu.edu/hbase/astro/expuni.html). Instead of using ...
3
votes
0answers
103 views

At 50% c, I go party to IC1101 galaxy, and back home. How many times can i go before the expansion of the universe will lock me on one side?

Longer version of the question: Let's say I'm an immortal being (I could be artificial) going to "party" to IC1101, one of the biggest galaxies of the universe, 1 billion light-years away ...
3
votes
0answers
70 views

Understanding Verlinde: How to get from emergent gravity to MOND

Verlinde ( https://arxiv.org/abs/1611.02269 ) tries to deduce MOND from emergent gravity. Can you help? Emergent or entropic gravity goes back to Jacobson. He starts with the entropy-area connection $...
3
votes
0answers
89 views

Is it possible to treat fermion reheating from inflaton decay only perturbatively?

In case of an inflaton Lagrangian $ \mathcal{L} = \frac{1}{2} (\partial_\mu \phi)^2 - \frac{1}{2}m_\phi^2 \phi^2 -h \overline{\psi} \phi \psi $ where the inflaton field is coupled only to fermions ...
3
votes
0answers
43 views

Entanglement and cosmological eras

I'm checking this paper: https://arxiv.org/abs/1405.7834 In the context of quantum cosmology, they assume that the universe evolves in an entangled state between the two epochs considered, i.e.$$\...
3
votes
0answers
51 views

Do redshifted galaxies with $z>1$ look like bigger because red light has a larger diffraction angle?

As a laser beam starts to diverge after a certain distance and has a divergence or diffraction angle which is largest for red light somebody could observe two laser sources distant 1m from each other ...
3
votes
0answers
146 views

Is the Universe 11 billion years old?

This clip claims scientists found a star 200M years older than the Universe. However, I took another assertion more seriously: scientists estimated a faster expansion rate of the Universe, driving age ...
3
votes
0answers
44 views

Would other parts of the universe have a different rest frame for their own Cosmic Background Radiation?

The CMB, once the Doppler shift from the Earth's motion is subtracted, is fairly uniform, which seems to imply that all the matter that emitted it moved at more or less the same velocity when it did. ...
3
votes
1answer
157 views

Changes to a Length of Physical Ruler Caused by Gravity vs Caused by Cosmological Expansion of Space

I read here (Feynmann Lectures, Lecture 42) that "Just as time scales change from place to place in a gravitational field, so do also the length scales. Rulers change lengths as you move around." (...
3
votes
0answers
60 views

How can the interior pressure of compact objects affect cosmology?

This paper suggests that dark energy concentrated in black hole interiors (they use an unconventional BH model) could act like a cosmological constant. Their claim is that to calculate the equation of ...
3
votes
0answers
61 views

What is the entropy of the cosmological horizon?

The Bekenstein-Hawking entropy for a black hole horizon is $S_{BH}=\frac{c^3 A}{4Gℏ}$. Is there a similar expression for the cosmological horizon?
3
votes
0answers
77 views

Could two atomic clocks really be used to detect gravitational waves from a distant source? If so, how?

Three articles report on the recent paper in Phys Rev. D: Flanagan, Éanna É. et al. 2019 Persistent gravitational wave observables: General framework (also ArXiv): Phys.org: Gravitational waves leave ...
3
votes
0answers
157 views

Is there any use of a quadratic equation of state in FLRW cosmology?

Consider standard FLRW cosmology. Usually, the relation between energy density $\rho$ and pressure $p$ of a cosmological fluid component is linear: \begin{equation}\tag{1} p = w \, \rho, \end{...
3
votes
0answers
39 views

Looking for a geometric cosmology model where time behaves like an orientation

This question is literally inspired from seeing the above scene unfold. Let the merging and splitting light spots you saw in the above gif be pairs of particles and anti particles, let the shape of ...
3
votes
1answer
124 views

What was the informational content of the universe at the Planck time?

If the energy of $1 \, \mathrm{bit}$ of information is $k_{\small{\text{B}}}T \ln{2}$, then the energy of that bit increases with the temperature of the system. When I try to calculate how much energy ...
3
votes
0answers
64 views

Functional freedom in cosmology. Why does it matter?

Roger Penrose's book "Fashion, Faith and Fantasy in the New Physics of the Universe" is a fascinating (and challenging) read, but while I understand his mathematical points about how additional ...
3
votes
0answers
34 views

Candidates for cosmic strings

What are the observed candidates for cosmic strings (or other defects), that are not ruled out? With references if possible.
3
votes
0answers
92 views

Are trans-Planckian field values a problem for the relaxion?

The relaxion is a new model-building gadget that solves the hierarchy problem by allowing the Higgs mass term to dynamically relax to zero. To describe it as simply as possible, leaving a lot out: we ...
3
votes
1answer
82 views

Why should LTB dust be comoving?

In many research papers about inhomogeneous cosmology, one often considers spherically symmetric (LTB) spacetimes where in the co-ordinate frame $(t,r,\theta,\varphi)$ wherein the metric assumes the ...
3
votes
1answer
197 views

Can we use pressure analogy to understand inflation?

Classically, the expansion of a gas in a container requires the gas to apply outward pressure to the walls of the container. Is it also true for the Universe during inflation? How did the inflation ...
3
votes
0answers
83 views

Metric vs coframe energy-momentum tensor in metric-affine gravity

Conventions Latin indices represent components in the anholonomic frame and greek ones are for coordinate components. I will call $R_{\mu \nu} := R_{\mu \rho \nu}{}^{\rho}$ (Ricci tensor) and $\bar{R}...
3
votes
1answer
58 views

Cosmological constant phase transition?

I recently watched at a talk by Cumrum Vafa in which he stated that the cosmological constant allows us to define a time-scale $T_\Lambda=1/\sqrt{E_\Lambda}$. The time scale of this time is about 10¹¹ ...
3
votes
2answers
222 views

Hints of pre Big-Bang physics and how to detect them?

The Big Bang model is the current large-scale accepted and tested theory. However, several new physics models or theories propose that there could be a previous Universe. To what extent can be test ...
3
votes
0answers
99 views

Hawking radiatiom from our cosmic horizon

Does our Hubble radius/cosmic horizon emit Hawking radiation just like an empty de Sitter space would emit from its horizon?
3
votes
0answers
93 views

Galaxies moving in fixed background or fixed in expanding background

Question: what is the most direct experimental evidence to suggest that the galaxies in the universe are fixed in an expanding spacetime, as opposed to moving in a fixed spacetime? Comments: The ...
3
votes
0answers
396 views

Adiabatic fluctuations

In Baumann's cosmology lecture, chapter 4, page 89, he defines adiabatic perturbation as: Adiabatic perturbations have the property that the local state of matter (determined, for example, by the ...
3
votes
1answer
232 views

Is Poplawski's “Cosmology with torsion” a “variable speed of light” cosmology?

In his 2010 paper "Cosmology with torsion", Poplawski mentions that, in the hypothesized formation of a new temporally-local universe within a black hole, its "expansion is not visible for observers ...
3
votes
0answers
102 views

Dynamics of the Inflaton field

The perturbations of the inflaton field $\phi$ are described by the following equation: $$\ddot\phi(x,t)+3H(t) \dot\phi(x,t)+\nabla^{2}\phi(x,t)/a(t)=-m^2\phi$$ where $a$ is the scale factor of ...
3
votes
0answers
363 views

Does any spacetime admit a global foliation in spacelike hypersurfaces?

In the comments of this question the following new questions came up: in general relativity, local coordinates can be found around any point, that single out a time coordinate and a three dimensional ...
3
votes
0answers
301 views

How to plot spectral indices $n_s$ and $r$ from Planck data?

From a certain inflationary model I have obtained the theoretical values for the spectral indices $n_s$ and $r$ as functions of $N$, the e-folds number of inflation. At this point it would be ...
3
votes
0answers
50 views

Where does this data comparing the CMB with Eddington's 'temperature of space' come from?

Eddington estimated the temperature of space assuming that star light would be scattered by interstellar dust. He came pretty close to the temperature of the CMB. http://www.astro.ucla.edu/~wright/...
3
votes
0answers
365 views

How to understand that Inflation generate almost scale-invariant perturbations?

Since during the inflation, the Hubble rate is approximately constant, the perturbations generated by inflation are almost scale-invariant. How to understand that?
3
votes
0answers
77 views

Basic question on Cosmic Strings

What is the typical size of Cosmic String Loops? If a loop is formed today, what are the restrictions on its size? I would like to know if there is a distribution of cosmic strings depending on its ...
3
votes
0answers
86 views

What would we observe as background on the sky if the big bang had never occurred?

The data we've received so far from satellites such as WMAP paints a near uniform distribution in intensity of the background radiation that we take as evidence that our universe had an origin, and ...
3
votes
0answers
163 views

How is translational symmetry related to Fourier decomposition?

The book (The Cosmic Microwave Background By Ruth Durrer) about cosmological perturbations says that because of translational symmetry of the background at a constant time, we can decompose our ...
3
votes
1answer
343 views

Momentum conservation in FRW spacetime

The spatially flat FRW metric in Cartesian co-ordinates is given by: $$ds^2 = -dt^2 + a^2(t)(dx^2 + dy^2 + dz^2)$$ As I understand it, since the metric does not depend on the spatial co-moving co-...
3
votes
1answer
395 views

Cosmological fluctuations: what is gaussian?

When we are speaking about gaussianity and non-gaussianity in a cosmological context, what is gaussian or non-gaussian in the CMB? What would a non gaussian CMB look like compared to a gaussian one? ...
3
votes
0answers
201 views

Power spectrum calculation for simple inflaton potential

I am trying to obtain an expression for the power spectrum $\Delta (k)$ of cosmological perturbations, see for example eqn (148) from the following TASI lectures. Right now, I would like to use the ...
3
votes
0answers
171 views

What role did the Higgs boson play in the Big Bang?

Scientists say that inflation stopped when a very peculiar energy was mobilized, did Higgs boson play any role in this? What is it's relation to the Big Bang? Some physicists, such as Michio Kaku, ...
3
votes
0answers
74 views

When Did Structures Begin to Orbit in the Universe?

In Big Bang Cosmology, I am familiar with the radiation era, and the moment of last scattering, but I am just not familiar with the physics of orbits, or of the “falling outward,“ if you will, that ...
3
votes
0answers
72 views

Active-sterile mixing for KeVins

People sometimes talk about KeV mass sterile neutrinos as a warm dark matter candidate. I think they call them KeVins (horrible name btw). Now, In order for it to be a good dark matter candidate it ...
3
votes
0answers
157 views

Avoiding Pseudo-tensors when addressing global conservation of energy in GR

Discussions about global conservation of energy in GR often invoke the use of the stress-energy-momentum pseudo-tensor to offer up a sort of generalization of the concept of energy defined in a way ...
3
votes
0answers
124 views

Should a radiation-filled Universe be scale invariant?

Imagine a spatially flat Universe, without cosmological constant, filled only with EM radiation. As Maxwell's equations without charges or currents are scale invariant then should this Universe be ...
3
votes
0answers
146 views

Dependence of the neutron-freeze-out temperature

What is the physical explanation for the dependence for the freeze-out temperature $T_\text{F0}$ of the neutrons as a function of the number of degrees of freedom $g^\star$? $$T_\text{F0}=\left(\frac{...
3
votes
0answers
130 views

Why can't Wetterich's “Universe without expansion” be tested compared to standard cosmological redshift?

I have read on several sites that this is a theory that cannot be tested. Why is this a theory that can not be tested? It seems to me that if the mass of particles is continually increasing, then the ...
3
votes
0answers
91 views

Are there any stable (not metastable) string theory compactifications with a positive cosmological constant?

Previously, on https://physics.stackexchange.com/q/86509/33806, I asked how we can be saved from false vacuum decay assuming we are living in a false vacuum. What if we are living in a stable vacuum ...

1
2
3 4 5
16