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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Could something theoretically observe the first light in the Universe?

Light travels at 299,792,458 meters a second. If you look at something that is 299,792,458 meters away, you're observing light from 1 second in the past. If an observer's distance from a light source ...
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How to find the maximum angle of a jet knot [closed]

Hey I am currently stuck with part (a) of this question, not really sure how to start so if anyone could give me any pointers to answer that would be great! Thanks
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1answer
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What is the extreme long-term elemental composition of the universe?

In Freeman Dyson's classic 1979 Time Without End paper he points out that, if proton decay does not occur, then normal matter will spontaneously fuse to iron on a timescale of $10^{1500}$ years, and ...
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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 ...
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Does the Big Bang have the absolute lowest amount of entropy? [duplicate]

We know that the event of the Big Bang has low entropy (and we don't know why), but... do we know for certain (or through convincing mathematical evidence) that the event of the Big Bang indeed has ...
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1answer
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Friedmann equations and different types of universes

I'm currently reading a book (in Danish) about the Friedmann equations and I have stumbled upon a third degree polynomial: $$ \Omega_\Lambda = \frac{4K_0^3}{27\Omega_0^2} = \frac{4(\Omega_0 + \Omega_\...
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1answer
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Restart the Universe expansion? [closed]

Is there anything that could restart the universe expansion? Basically a black whole or something else entirely different? It seems to me that, once the Universe stops expanding, the human life ...
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1answer
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Does Hubble constant $H$ change with time in $N$-body simulations?

In $N$-body simulations, there is a Hubble constant $H$ in equation of motion. when we integrate the equation of motion, does this constant H also change with the simulation time? or it is just a ...
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Why is the universe expanding yet remaining flat?

According to The Big Bang Theory everything started from nothing and suddenly started expanding creating everything that is. So why is it only expanding on an X&Y axis? I can only find facts that ...
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Does Olber's paradox imply a bright sky or a divergent one

Doing Olber's paradox with specific systems is simple enough. Take stars on a grid of length $\ell$, then the total luminosity is just \begin{equation} L = \sum_{i,j,k} \frac{L_0}{4\pi\ell^2(i^2+j^2+...
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Amplitude for neutron-proton conversion from first principles

I'm reading Dodelson's Modern Cosmology, and in one of the exercises on Big Bang nucleosynthesis it is quoted without reference that the amplitude for neutron-to-proton conversion is given by $$|\...
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Would the nightsky be bright (filled with starlight) without accelerating space expansion?

As I currently understand, if a photon was emitted from a far away point in the Universe, beyond the event/particle horizon, so that the space inbetween the emitter and us is expanding faster then ...
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Vacuum energy density with respect to what reference frame?

Wikipedia quotes the observed vacuum energy density to be around $10^{-9}\frac{J}{m^3}$. Whatever the real energy density is, my question is, with respect to what reference frame is this being quoted?
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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-...
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How would one translate free-in and freeze-out for dark matter and cosmology in french? [closed]

Remark : I have understood the concepts. My question is only on semantic, for translation in french of the two words "freeze-in" and "freeze-out". The freeze-out corresponds to dark matter in ...
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1answer
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What happens in the distant future of the universe?

In the standard cosmological model LCDM the universe approaches the asymtotic deSitter state in the distant future. Is it correct to say that all light-sources situated on the ultimate lightcone of a ...
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Does the FRW metric imply spacetime scales?

The FRW metric can be written in conformal co-ordinates to give: $$ds^2=a^2(\eta)(-d\eta^2+d\mathbf{\Sigma^2}),$$ where $\eta$ is the conformal time and $\mathbf{\Sigma}$ ranges over 3-dimensional ...
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2answers
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Is the photon velocity regarding a point in a expanding universe constant?

Could be a photon moving in an expanding universe be thought of as an ant moving along an elastic rope fixed at the upper end on a cieling that is streching due to a constant mass at its bottom end. ...
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1answer
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Brans-Dicke vacuum cosmology?

Has anyone heard of papers describing Brans-Dicke vacuum cosmology solutions? I'd be interested to know if the following calculations make sense. The Brans-Dicke action for a Universe dominated by ...
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2answers
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How do we know that the universe is really fine-tuned? [duplicate]

How physicists come to the conclusion that the cosmological constant and the other constants are really fine-tuned in a way that if they are changed just a bit, then stars and life won't exist?
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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.$$\...
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3answers
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Expansion of space or contracting of matter? [duplicate]

Can the expansion of the Universe effect be explained by contraction of the overall matter instead? That is, all material objects are in fact contracting, due to what the universal space seems (and ...
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How can the universe have a finite information capacity yet be continuous? [closed]

The information capacity of the universe is proportional to its surface area, so the observable universe has a finite information capacity. But the universe is also usually thought to be infinitely ...
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Dimension of scale, or depth - does it exist?

Description of an object in 3 standard space dimensions seems not to be its full description. Say, the placement of a molecule is described by $x=2N$, $y=4N$, $z=7N$ measurement units in the Descartes ...
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Does the Many worlds interpretation of quantum mechanics imply the existence of all conceivable worlds? [closed]

what I mean by this is, does every universe conceivable exist? would any universe i think of exist? could there be a universe of nothingness? are there universes of TV shows? are there universes with ...
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1answer
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Is the decoupling matter/radiation in the universe evolution due to the electron/nuclei recombination or to the *density* decrease or to *both*

Is the matter/radiation decoupling in the evolution of universe due to -the electron/nuclei recombination (because temperature decreases), so that photon-matter cross-section reduces since there is ...
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1answer
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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 ...
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3answers
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Farnes, cosmology involving negative mass

I came across a paper recently which seems to be getting a lot of popular press, and a student also emailed me to ask about it: -- Farnes, "A Unifying Theory of Dark Energy and Dark Matter: Negative ...
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Was helium hydride really the first molecule?

This press release by NASA: https://www.nasa.gov/feature/the-universe-s-first-type-of-molecule-is-found-at-last/ When the universe was still very young, only a few kinds of atoms existed. ...
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Radiation Density Parameter Today

I would like to insert in my thesis the value of the radiation density parameter today with its appropriate error and I was checking the latest Planck results here. I am a bit confused since I thought ...
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3answers
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Does light from source one mil. light years away take more than 1 mil. years to reach us?

Because of the expansion of space itself, did light from a galaxy one million light years away actually take more than one million years to reach us? Also, if we are looking at a galaxy one billion ...
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Why was helium hydride ion HeH$^+$, not hydrogen H$_2$, the first molecule formed in the early universe? [duplicate]

Why was helium hydride ion HeH$^+$, and not hydrogen H$_2$, the first molecule formed in the early universe? How did NASA scientists confirm it? https://www.nasa.gov/feature/the-universe-s-first-...
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How to Change Coordinate Systems in General Relativity

Let me preface by stating that I have no experience with General Relativity. I am working on a project for school that requires a little knowledge of it, so I am hoping to find some help. I do have ...
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Other explanation for cosmological redshift?

I'm interested if any of the following explanations have enough predictive capability to explain the observations we see today. The claim is that the Universe is not expanding, and that red-shift of ...
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3answers
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Alternative to Dark Energy

If Black Holes consume mass and re radiate it in the form of massless photons the mass of the universe is declining. Could this explain the accelerating expansion of the universe without the ...
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1answer
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Density fluctuations of the present Universe with Large scale structures

Disclaimer The question below is based on a vague knowledge, and hence, statements can be potentially wrong or misleading. An important quantity in Cosmology is the density fluctuation of matter ...
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Why is the homogeneity problem actually a problem?

I don't mean to be flippant but I have never understood why the homogeneity in the universe needs.explaining. I have searched other questions on stack exchange and they don't really address the ...
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1answer
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Spacetime diagram of LCDM

I have a question concerning the LCDM spacetime diagram https://i.stack.imgur.com/Uzjtg.png published on the Physics Forum Stack Exchange website Can space expand with unlimited speed? How are the set ...
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1answer
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How are the dark matter (DM) thermalization rates derived in singlet DM model?

This well-cited paper talks about a minimal renormalizable extension to the Standard Model (SM) to incorporate particle dark matter (DM) into it by adding a real scalar field $S$ which (unlike the ...
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How does the baryon asymmetry control temperature fluctuations of CMB?

The temperature fluctuations of the Cosmic Microwave Background (CMB) have a sensitive dependence on the quantity of baryon asymmetry of the universe. In fact, analysis of CMB fluctuations is one of ...
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2answers
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What is the interpretation of a wave function of the Universe in Hawking's no boundary proposal?

In the path integral formalism we have an in state $\Psi_{in}[\phi]$ and and out state and we find the amplitude for going from one to the other: $$\Delta[\Psi_{in},\Psi_{out}] = \int \Psi_{in}[\phi]...
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How does the CMB constrain the baryon asymmetry?

The CMB contains information about baryon acoustic oscillations in which baryons (I assume protons and electrons) and photons form a plasma exhibiting sound waves. How is information about the baryon ...
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1answer
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What are “the background equations” in cosmology?

We're currently working on perturbations within cosmology. There is something I have not heard before which has cropped up, that is: a reference to the term "the background equations". Are these just ...
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Is the fluctuation pattern of the Cosmic Microwave Background (CMB) identical in every location of the universe?

I know that the Cosmic Microwave Background (CMB) is the leftover radiation from the "surface of last scattering". Let's say an alien civilisation lives on andromeda or further away. Would they see ...
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2answers
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Is the relativistic Doppler effect the only way to show that the universe is expanding?

Is there another way to show that the universe is expanding, rather then only relying on the relativistic Doppler effect?
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What does an outgoing null geodesic for FLRW metric look like?

I know it involves setting the line element to zero, as well as setting $d\phi$ and $d\theta$ to zero. But other than that I do not know what to do. I get an expression involving $a(t)$, the scale ...
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Hamilton Constraint of the WdW equation

Can someone explain specifically what the surface term of the hamilton constraint in quantum cosmology actually describes and how it creates time even though we start with a timeless universe? And why ...
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What is the advantage of using Kruskal coordinates?

(as opposed to Eddington-Finkelstein coordinates) The EF coordinates already take care of the coordinate singularity so I dont see a point for using Kruskal coordinates.
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On average, how many atoms are in there in a molecule?

I was doing some Fermi style questions and one of them got me thinking about how many atoms are in the average molecule. Since hydrogen is the most abundant, and hydrogen is diatomic I guessed around ...
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Infinite universe and the impossibility of monochromatic light and infinite plane waves

I recently asked a question relating to this excerpt from the textbook Optics, fifth edition, by Hecht: Mathematically, the plane wave extends out to infinity in all its directions, and, of course, ...

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