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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.

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Physical interpretation of FRW normal coordinates

The Friedmann-Robertson-Walker metric (I consider for notational simplicity the flat space case): $$\text d s^2 = \text d t^2 - a(t)^2\text d \boldsymbol{x}^2$$ can be brought to normal (Minkowski) ...
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8 views

What is the NP form of Rarita-Schwinger equation

Does anyone knows how to write down the Rarita-Schwinger equation in Newman-Penrose formalism?
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34 views

Total number density of galaxies and problematic expression in practise

I am asked to give the formal expression of the total number density of galaxies and explain why is this expression problematic in practice. From what I saw from my research and into my lectures, I ...
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44 views

Can we make a cellular automata model for the universe with no rules? [on hold]

There are models where the universe is described as behaving as a cellular automata. For example, nobel laureate Gerard 't Hooft is one of the proponents of these models (he has created a series of ...
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2answers
33 views

Intuition for why matter dominated expansion is faster than radiation domination?

In a matter dominated universe $a_{\rm mat.}(t)\sim t^{2/3}$, while in a radiation dominated universe, $a(t)_{\rm rad.}\sim t^{1/2}$. Therefore, a matter dominated universe is expanding more quickly, ...
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1answer
19 views

What condition needs to be met before a type of matter dominates the density of the universe

If we're given some theory about the expansion of the universe such as particles being created at some time t and having a density related to their critical density at that time for example and we ...
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12 views

Expansion In Electromagnetic field

Will electromagnetic field affect the expansion of the universe? If we consider the stress tensor we must add the one for electromagnetic field in the perfect fluid stress tensor. For this stress ...
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2answers
53 views

Is gravitational lensing affected by dark energy?

Is gravitational lensing affected by dark energy? I mean, despite the effect if cause on the expanding Universe, could dark energy cause gravitational or anti-gravitational lensing?
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1answer
64 views

A Universe From metastable false vacuum [on hold]

https://arxiv.org/abs/1404.1207 This paper assumes that our universe was created out of nothing . This paper says that the quantum fluctuations in the metastable false vacuum created a true vacuum ...
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Where did the energy from the CMBR go? [duplicate]

Basically the title. The wavelength of light from the CMBR has expanded as the universe expanded and so it has lost energy since it is inversely proportional to the wavelength. Where did this energy ...
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2answers
36 views

Does the pattern of CMB anisotropies change over cosmic time?

Currently, the Universe is about 14 billion years old, and we now see a certain pattern of anisotropies in the cosmic microwave background (CMB). If we wait another 14 billion years, and repeat the ...
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1answer
40 views

Is the speed of light constant in time as well as space? [closed]

What I mean by that is that if we measure the speed of light against comoving distance instead of proper distance, is it still constant? Since comoving distance doesn't vary with time that would make ...
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1answer
52 views

Does a level I multiverse implicate a very long life? [closed]

Let's suppose that there are infinite level 1 universes, agreeing with physicist like Max Tegmark and even before them with philosophers like Giordano Bruno. Now, please follow this thought ...
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64 views

Negative Mass and Gravitation vs. Inertia

Recently an article came out about an interesting result in cosmology that somehow "unifies" dark matter and dark energy. It talks about a "creation" tensor that describes negative mass appearing ...
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0answers
19 views

Frame of reference for simulation of a universe [closed]

Nowadays, we can simulate our universe in the computers. When we are observing the simulated universe, are we in an inertial or non-inertial frame of reference wrt the simulated universe? What ...
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0answers
20 views

Energy conservation in the Universe [duplicate]

When the Universe expand CMB photons lose energy because each photon has its wavelength getting augmented. This represent a loss of energy for each photon in the Universe as the Universe expand. Where ...
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36 views

Where the universe is situated? [duplicate]

I have heard a lot about the expansion of the universe. Many theories say about the expansion of the universe. But i have three questions. Where this universe is situated? As our theories specify ...
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0answers
29 views

What are exactly event networks? [on hold]

I found this draft of an article which I found very interesting (http://www.goertzel.org/papers/eventnet.html) After reading the article a few times, I think I have a clear picture of what does it ...
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0answers
23 views

Could gravity and dark energy be the same thing? [duplicate]

I don't yet have enough reputation to comment, so I'm asking the question anew. I've read over some of the other answers, and have some issues with them. I have seen descriptions of the effects using ...
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1answer
57 views

Is this how vacuum works?

Vacuum exerts a net zero force, or i guess pressure? on what's inside it, right? So if the universe is expanding at an accelerating rate, is the vacuum of outer space is growing? Like space is ...
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28 views

What are the differences and similarities between Wheeler's it from bit and holographic principle? [closed]

Are these both concepts the same? Or they are extremely different? Also, since they are both related to the multiverse hypotheses are they multiverse theories as such? (In the book "Closer to the ...
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2answers
62 views

How can the red shift due to Hubble flow be consistent with the law of conservation of energy? [duplicate]

Given that there is no change in relative kinetic energy attributable to the expansion of the universe, what accounts for the difference in energy emitted vs energy absorbed (i.e. the red shift ...
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2answers
53 views

Why does the Drake equation never account for the abundance of the elements in the universe?

The Drake equation which is used to describe a rough estimate of possible ET civilizations that is able to communicate with us does not take into account the abundance of the chemical elements in the ...
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Entropy and observers

Struggling to understand the stance Bousso takes in his paper, The Entropic Landscape, https://arxiv.org/abs/1001.1155 Is he saying that because only by observing an increase in available state we ...
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0answers
23 views

Released energy through bubble nucleation

I am reading the paper by Coleman "Fate of the false vacuum: Semiclassical theory" which explains how bubble nucleation occurs in first-order phase transition in field theories. I will summarise the ...
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1answer
39 views

I've read a cosmic string can accelerate a mass. Can someone explain how it works?

In the book Brief Answers to the Big Questions: the final book from Stephen Hawking Hawking describes cosmic strings as follows; Actually they are more like rubber bands because they are under ...
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2answers
53 views

Can general theory of relativity predict the scale of the universe (finite or infinite ) [closed]

Can GTR predict boundary of the universe . GTR is too difficult for me to understand its main purpose and its contribution in cosmology . i have another question to be asked -I have been struggling ...
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3answers
68 views

Absorbing the Cosmological “Constant” in the standard Energy-Stress Tensor

Recently I found some publications on Cosmologies with variable cosmological constant. The Bianchi Identity then implies that the divergence of the modified Energy Stress: Tensor $$\hat{T}_{ab}=T_{ab}...
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1answer
162 views

Where is all the Antimatter?

This is obviously a duplicate of If matter and antimatter... However, it's now four to five years after a swarm of videos were made on this subject. At the time, there was a consensus that the LHC ...
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1answer
88 views

Milky Way Density

It seems to be a simple question, but I wasn't really able to find an appropriate answer: How dense is the Milky Way? I am certain that there are reliable statistic, maybe even new ones from the GAIA ...
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1answer
79 views

Hawking vs Susskind Information Paradox PBS Nova TV show - nowhere to be found [closed]

I remember there was an awesome Nova episode, at least 10 years ago, where there was a battle in which Susskind ridiculed Hawking for his silly theory of violation of the Conservation of Information ...
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2answers
151 views

Big Bang: what is it exactly that is expanding?

I have read a bit about the Big Bang over the years, but being no physicist I have never been able to really understand what it is about. As far as I know, starting with Hubble we have been able to ...
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3answers
165 views

If the expansion of the universe were to stop, would galaxies continue moving away from each other at a constant speed?

Consider a simple universe consisting of two very distant galaxies (neglect gravity between them). The relative motion between them is such that, if the universe were not expanding, they would be at ...
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1answer
51 views

Numerical solution to Mukhanov-Sasaki equation

I am trying to figure out how to solve the Mukhanov-Sasaki equation to compute the power spectrum of an inflation potential that exhibits an ultra slow-roll phase that gives rise to an enhancement in ...
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1answer
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How can I understand the reaction rate of WIMP dark matter intuitively?

I'm reading a cosmology textbook(sorry it's Japanese) and stacked at statement that If the mass of WIMP is enough smaller than 100GeV, the reaction rate is represented as $\langle \sigma_a |\mathbf{\...
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0answers
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Possible origins of the Antlia 2 galaxy?

At $32.3\,{\rm mag}\,{\rm arcsec}^{-2}$, the Antlia II dwarf galaxy is the most diffuse galaxy ever observed in terms of surface brightness. It was only discovered by pre-selecting stars based on ...
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1answer
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What will be the expression between conformal time and scale factor for $\Lambda$ CDM model (for $k=0$, $\Omega_M + \Omega_\Lambda = 1$)?

An approximation of the relationship between the scale factor and time for our universe is given by the following expression: $\displaystyle a \approx \left (\frac{\Omega_{M_0}}{\Omega_{\Lambda_0}}\...
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1answer
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Effective low energy actions in string cosmology

I wonder why can SUGRA actions be used in string cosmology, since in some cases they are used to model big bang scenarios (for example the pre big bang scenario in chapter 4 of Gasperini's "Elements ...
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Does the universe has a center like a real explosion? [duplicate]

I don't really know much about cosmology but I was thinking one day about the center of the universe. Now 'Big Bang' was the birth of our universe. Then Shouldn't the place (or region of space) where ...
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How does the redshift - distance graph show the rate of expansion of the universe at every moment in time?

By plotting the graph of redshift against the distance of the object from earth, we are able to obtain a best-fit curve showing the relationship of redshit against the distance. How does this ...
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1answer
49 views

How was electrical charge formed in the Big Bang?

We know that fundamental particles (Quarks, electrons, etc.) were formed a while after the Big Bang. How was their electrical charge (its value and sign) determined? Was electrical charge present in ...
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13 views

Dark Matter- Simplifying WIMP abundance relation

I am currently studying WIMPs and how/why they are a candidate for dark matter. I am reading "Cosmology and Particle Astrophysics"~ Bergstrom and Goobar and it states the following 'abundance' ...
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2answers
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Infinite Speed of Light

I recently watched a video that stated that Newtonian Mechanics assumed an infinite speed of light. That same video, "PBS Space Time: The Speed of Light is Not About Light", stated that if the speed ...
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92 views

Meaning of the simplest potential of quintessence models. Fields in denominator?

I am reading Sec. 1.12 of the Cosmology book by Weinberg. In this section he explains the very simple model of quintessence which attempts to provide a dynamical explanation of the smallness of the ...
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1answer
64 views

Chemical equilibrium for a $2-2$ scattering and Boltzmann equation in cosmology and astroparticle physics

Definition of chemical equilibrium According to the definition of chemical equilibrium in Wikipedia, it is a situation where the rate of the forward reaction is same as the rate of backward reaction ...
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1answer
68 views

Spatial part of Robertson-Walker metric

The spatial part of the FRW metric can be written as $$d\Sigma^2=d\rho^2+f^2(\rho)(d\theta^2+{sin}^2\theta d\phi^2)$$ where $f(\rho)$ satisfies $$\frac{df}{d\rho}=\frac{f(2\rho)}{2f(\rho)}.$$ I am ...
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2answers
68 views

Rindler Metric and Minkowski metric

I am trying to understand why the Rindler Metric line-element and Minkowski metric line-element represent the same spacetime. Could someone help me understand that?
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1answer
44 views

How did inflation affect time-like space-time?

Alan Guth gave an overview of inflation in a series of videos for World Science U. The emphasis is on how inflation expanded a patch of space to the size of a marble, with no mention of how time-like ...
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1answer
59 views

Derivation of Boltzmann equation in Kolb and Turner's The Early Universe

In the homogeneous and isotropic FRW Universe, the collisionless Boltzmann equation is given by $$ E\frac{\partial f}{ \partial t}-\frac{\dot{a}}{a}|\textbf{p}|^2\frac{\partial f}{\partial E}=0\tag{1}$...
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1answer
69 views

What is the cause of the vacuum of space?

It seems logical that if space is expanding and also being dragged into black holes throughout the universe, the result would be a vacuum in the universe. Is that a satisfactory explanation of why ...