Questions tagged [cosmology]

Questions about the origin, history, evolution and fate of the Universe.

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1answer
29 views

Would stars have formed in the Universe if atomic hydrogen couldn't make molecular hydrogen?

I just saw this YouTube video made by PBS Studios. There they explain that $H_2$ can reduce the traslational kinetic energy of regular $H$ atoms when it is formed since the energy can also be stored ...
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Is the Universe infinite? [duplicate]

https://en.wikipedia.org/wiki/Universe Is the Universe infinite or finite ?
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What features are different in old/young galaxies, what can be seen?

As you all know, the further the galaxy is from us, measured by redshift, the younger it is supposed to be, but what other features can be seen in those younger galaxies to confirm their age? Hints: ...
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1answer
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Why dimmer high-redshift supernovae means the expansion is accelerating, if the dilated region pertains the distant past?

I must be seeing this wrong, because it seems to me the data indicates faster expansion in the past, contrary to Adam Reiss' study that led to the birth of the notion of 'dark energy'. In Reiss' ...
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32 views

Relation between photometric redshift in a given bin and conditional probability

I take over an old question that is not yet clear for me. Indeed, I would like to formulate the equation (1) below under the form of "conditional probability" but I can't manage to do that. ...
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2answers
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How are younger objects outside our observable universe when the older CMB is an observer's temporal “edge”?

We know the observable universe is limited to objects whose light has had time to reach us and that an observer very far away will see a slightly different (assuming cosmological principle) observable ...
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1answer
109 views

What was the size of the universe around 400 million years after the Big Bang?

I was trying to find out how large the universe was around 400 million years after the Big Bang as this is when some of the first galaxies were formed but I am having trouble finding out the answer or ...
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1answer
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What are GEODEs?

Phys.org's Researchers predict location of novel candidate for mysterious dark energy says that GEODEs may explain the dark energy problem. I'm not an astronomer but I've never heard of GEODEs. I know ...
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spectral index of primordial power spectrum

According to eq (7.62) here, the dimensionless primordial power spectrum takes a power law form: $$\mathcal P(k)=A_s\left(\frac{k}{k_0}\right)^{n_s-1},$$ with $n_s\approx 1$, i.e. $\mathcal P(k)$ is ...
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3answers
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Cosmological redshift vs doppler redshift

I'm reading Harrison's "Cosmology: Science of the universe" because Harrison focuses on the distinction between cosmological redshift (he calls it expansion redshift) and the Doppler redshift. He ...
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Is there any way that we can see the signs of Cosmic Big Bounce of Loop Quantum Gravity, in the Cosmic Microwave background?

Today, Quantum Cosmologies' biggest questions are that what happened before the Big Bang?? Some say, that we can actually have a certain sort of clue with Gravitational Waves to see exactly that what ...
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1answer
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What does “located in the Hubble flow” mean?

I do not understand that much about the term Hubble flow. For cosmological studies with type Ia supernova, I often see the sentence like "We used 100 SNe Ia in the Hubble flow". What does ...
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Does the critical density of the Universe also include Dark Energy?

It is often said that it seems that the Universe is quite close to critical density (for example https://astronomy.swin.edu.au/cosmos/C/Critical+Density). Does this include Dark Energy as well (apart ...
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What is dark energy made of? [duplicate]

What are the fundamental constituents of Dark Energy? Can we quantize dark energy in the first place, if it is a kind of Energy?
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119 views

What happened before, Big Bang or inflation?

I always thought that the cosmic inflation happened after the big bang (10-36 sec) and lasted for a very tiny fraction of time). Now, I recently came across a couple of articles (links below) claiming ...
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1answer
115 views

The energy density of DM (to be fluid,state parameter=-2)increases with time. Does it violate the concept of accelerating universe?

We know that dark energy with value of state parameter $\omega<-1$ is called phantom dark energy model. In this model if we assume dark energy to be a fluid with value of state parameter $\omega=-2$...
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1answer
499 views

How to correct observed flux densities for redshift

Say I have a spectrum of a galaxy at a redshift $z$, in flux density units of erg/s/cm^2/Angstrom. I'd like to recover the spectrum (in the same flux density units) at z=0, i.e. at its rest ...
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1answer
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Approximate formula to find velocity from cosmological redshift

From IOAA 2013 (Greece) Theory question no. 15, they stated that the approximate formula to find velocity from cosmological redshift is $$v = c*log_e (1+z)$$ and that it is often used by cosmologists. ...
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Gravitational waves and gamma ray burst: how were the error bars determined for this speed of gravity calculation? Was $H_0$ used?

This newly updated answer to How precise are the observational measurements for the speed of gravity? and this answer to How is the most accurate value of 𝐺 measured? cites the November 2017 arXiv ...
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1answer
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Is cosmology an accurate field? [closed]

I'm new here this is my first question. I'm an engineering student and I really like mathematics, I've recently watched a movie about Stephen Hawking (The Theory of Everything) In which he makes ...
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1answer
81 views

Puzzle about proper distance

So I tried deriving the radial co-moving coordinate formula to an observable galaxy for which the light observed at time $t_o$ was emitted at time $t_{em}$. i.e $r_0=\int_{t_{em}}^{to} c/a_{(t)} dt$ ...
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Demonstration of scales for the two components (transverse and perpendicular) of Baryonic Acoustic Oscillations (BAO)

I transfer here a post that has not had any answer pn physics exchange, so I am going to delete this latter. Maybe I will have more luckiness here. In an astrophysics context about BAO (Baryonic ...
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1answer
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What exactly is a “conclusive association” in Astronomy?

Phys.org's Astronomers use slime mold model to reveal dark threads of the cosmic web says A computational approach inspired by the growth patterns of a bright yellow slime mold has enabled a team of ...
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Get the expression of probed volume between 2 redshifts

1) I can't manage to find/justify the relation (1) below, from the common relation (2) of a volume. 2) It seems the variable r is actually the comoving distance and not comoving coordinates (with ...
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1answer
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Basic explanation of baryon acoustic oscillations; what if anything is actually oscillating?

Sci-News' Sloan Digital Sky Survey Collaboration Releases New 3D Map of Universe links to the two videos The eBOSS 3D map of the Universe SDSS releases largest 3D map of the universe ever created ...
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1answer
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Calculate object apparent diameter by distance and radius [duplicate]

I'm trying to create a dynamic starfield for game using the HYG database. Because there are no data about the apparent size of the stars from earth, I will need to get it myself. I found a formula ...
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1answer
43 views

Would DCBH form supermassive stars first before BH formation

In reading some literature it seems ambiguous (intentionally?) as to the path that Direct Collapse Black Holes (DCBH) take. Some papers do talk about pristine Halos collapsing to form supermassive ...
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Transition of Domination in the Universe

While deriving the epoch of matter-radiation equality, we assume the transition from radiation dominated universe to matter dominated universe as instantaneous. But how instantaneous is this ...
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1answer
51 views

Metal lines in Lyman-alpha forest

I need to find metal lines in the Lyman-alpha forest of quasar spectra, without the red side of lyman alpha emission line. Can you guide me how to do this?
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1answer
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Expresion of comoving distance

I have a simple question : How to prove the following relation : The comoving distance to an object at redshift $z$ can be computed as $$r(z)=\dfrac{c}{H_{0}} \int_{0}^{z} \dfrac{\mathrm{d} z}{E(z)}$...
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1answer
40 views

Which redshift is used to determine the Hubbleconstant?

I think they measure cosmological redshift to use in the Law of Hubble-Lemaître together with the distance to calculate $H_0$. Is this correct, or do they use Doppler shift (too)? $H_0$ indicates how ...
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1answer
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Can the electromagnetic counterpart of a gravitational be seen by 'small' telescopes?

As follow-up question for my previous question I'm thinking about the search for the electromagnetic (EM) counterpart of a gravitational wave (GW). Is it possible to search for those events with '...
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Does a merging massive binary black hole ‘emits’ more than one gravitational wave?

If we want to use gravitational waves (GW) to determine the Hubble constant, we need to find the source in the electromagnetic spectrum (EMS). However, we need to be lucky to ‘see’ it simultaneously ...
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Does the cosmological principle apply to the entire universe?

Some cosmological models assume that the universe is isotropic and homogeneous and that is also flat and infinite. If the universe is infinite now it was infinite immediately after the big bang. If ...
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1answer
341 views

Why are quasars so far away that they couldn't be optically resolved in the 1950's? [duplicate]

Wikipedia's Quasar includes the following in its introduction: The term quasar originated as a contraction of quasi-stellar (star-like) radio source, because quasars were first identified during the ...
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Does time flow in a Minkowski spacetime?

In a spacetime where the stress-energy tensor is null (no energy, no matter, no entropy), I wonder whether any motion of the variable time in the phase-space is well defined. The arrow of time, meant ...
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Pivot scale in cosmology, CMB observatiion

What is a pivot scale, pivot frequency in general, and particularly in the cosmology? I don't get the idea. Almost everywhere I found the same the same sentence "we take the standard value of a ...
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Is cosmological redshift caused due to expansion of space or distance traveled by light?

If the universe wasn't expanding, would light still be redshifted after traveling some distance? Is there a way of being sure that it's the expansion that causes light to redshift and not just a ...
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67 views

If time is not absolute and depends on the gravitational field, then why we keep using our own (particular) time in cosmology?

Sorry if my question is not really clear, but indeed it is not clear in my mind neither. But maybe some of you can help. Time is not absolute and can be affected by the gravitational potential. Now ...
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1answer
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Bounded Observable Universe and General Relativity

The book General Relativity for Mathematicians by Sachs and Wu defines a "spacetime" as a connected 4-dimensional, oriented and time oriented Lorentzian manifold. I emphasize the word "connected." ...
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How is the Hubble constant determined from gravitational waves?

We know there is a discrepancy between measurements of the Hubble constant, $H_0$. On one side there is the method of the Planck mission, where they use the CMB and the $\Lambda$CDM model to determine ...
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1answer
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'Little h' usage in cosmological simulations

I am running a cosmological simulation and am having some trouble putting things into code units. The physical distance units in my simulation are in terms of $\text{Mpc/h}$, where $h$ is the ...
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1answer
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Is our universe a singularity?

According to the Big Bang theory our universe used to be a lot smaller in size. It actually used to be so small that in the beginning it used to be a singularity. And the universe started to expand ...
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How does the evolution of a solar system not break the second law of thermodynamics?

Please forgive: I am a layman when it comes to physics and cosmology, and have tried finding an answer to this that I can understand, with no luck. As I understand it, the solar system evolved from a ...
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How to derive the formula of magnitude error caused by peculiar velocity?

I want to understand how to derive the formula of magnitude error ($\sigma_m$) which is caused by peculiar velocity of a galaxy, like this $$ \sigma_m = \frac{5\ \sigma_V}{cz\ \rm{ln(10)}} $$ ,where $\...
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Astronomy A Physical Perspective [closed]

Is "Astronomy: A Physical Perspective" by M. Kutner written in 2003 a good introductory textbook? What other textbooks could you recommend? I want to study astronomy on my own but with scientific ...
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1answer
64 views

Neutrino Modelling in Friedmann Equation

so I'm trying to model neutrinos in the Friedmann Equation. I've covered the case of the Benchmark Model where we have matter, radiation, curvature, and the cosmological constant, Lambda. I know my ...
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Paper recommendations for career guidance

I'm an undergraduate student in astronomy and I'm one year from graduating, but I feel I still can't choose what career path I want to pursue. To date I've only had two courses of proper astrophysics ...
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1answer
154 views

Assumptions necessary for the strong form of Olber's Paradox

From Wikipedia, here's Lord Kelvin's statement of Olbers' Paradox: Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by ...
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158 views

What good is the Einstein de-Sitter model?

Why is the EdS model still taught and used to predict the age of the universe if ours is not an EdS Universe. Does this model have any pros?

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