Questions tagged [cosmology]

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

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Historical estimates of the density parameter

From the reference in this answer I learned that our current estimate for the density parameter (i.e., the density of the universe divided by the critical density, which determines the shape of the ...
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I need to study space, planets and black holes, how do I study them on my own [closed]

I need to study space, planets and black holes, how do I study them on my own, knowing that I am an Arab and I do not know English. I use translation applications. I speak with you. Can you help me ...
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Optical emission line data archives

I am trying to do exhaustive optical emission line analysis of some active galaxies using archival data. What are some of the publically available data archives from where one could obtain optical ...
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Researching mechanics without “fixed stars”

In the history of humanity, easily observable extra-(Solar System) objects greatly helped understanding certain phenomena inside the Solar System. Importantly, the “precession of the equinox”, and ...
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Computing the over-density of Lyman-alpha emitters

Let's assume that I have observations of $N$ galaxies in a projected area $A$, within a redshift range $\Delta z = z_{max} - z_{min}$. What is the correct way of computing their number volume density? ...
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Do objects lose momentum as space expands

As far as I know, photons' wavelengths can be considered increasing as space expands, making them lose energy and momentum. Does the same apply to physical objects? I understand a photon's speed is ...
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If rotation has nothing to do with gravity, why do astronomers always link swirling dust and gas to stellar evolution?

All my life I was taught in school that we have gravity because the earth spins. Now I know that is not true. Online academics were baffled why anyone thought so. I can only say that's what they ...
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How, exactly, does the precise measurement of the CMB's polarization modes and temperature fluctuations tell us the value of Hubble's 'constant'?

Amidst all the talk a year and change ago about the value of the Hubble parameter reached by the Planck satellite team, and how it's value differed from the value reached by the 'distance-ladder' team(...
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What are the pros and cons of angular cross-correlation in real space and harmonic space?

In observational cosmology people often measure and model cross-correlations between different tracer maps. There are generally two ways to measure cross-correlations: in real space (two-point ...
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Cosmological density fluctuation dependence on scale factor

I want to show that in the radiation-dominated era, $\delta_H^2 \propto a^{1-n}$. I know that the wavenumber corresponding to the Hubble scale goes like $a^{-1}$ and that $\delta_H^2 \propto k^3 P(k)$ ...
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From what distances do the atoms in you come from?

If the atoms of a human come from stars, comets, nebulas and magnetrons, then what is the greatest distance that two atoms of a human can possibly have been away from each other previously? Perhaps ...
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Could there really be a preferential direction to the speed of light?

This question is inspired by a recent video by Veritasium Why no one has measured the speed of light linked below. To summarize the video, Derek points out that it is impossible to measure the one-way ...
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Transverse Comoving distance

I don't understand the link between the comoving distance and transverse comoving distance : how can they be equal ? Here an example of definition that I have found : Angular Diameter Distance : The ...
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Clarifications about distances in cosmology

I would like to get clarifications about some usual notions of distances in cosmoogy. First, the comoving distance is the current distance of objets whose light ...
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Question on the singularity theorem

I have just started studying Cosmology and we have been asked to prove that in an expanding FRW Universe which obeys the strong energy condition: $$\rho + 3P >0$$ Then there must exist a Big Bang ...
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Why does the Hubble flow does not carry photons with it?

I have this naive question. We know that the expansion of the universe "takes" or "carries" the galaxies with it, separating them, but this doesn't apply to photons. My guess is ...
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Cross-correlations between Lagrange and Fourier synthesis

In the context of forecast for large surveys, I have to make cross-correlations between 2D (with angular coordinates of Lagrange transformation for GC photometric and Weak Lensing) and 3D (Fourier ...
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What is the 'scale factor' equation for a dark-matter dominated universe?

The Friedmann equations can be solved exactly in presence of a perfect fluid with equation of state $${\displaystyle p=w\rho c^{2},}p=w\rho c^2$$, where ${\displaystyle p}$ is the pressure, ${\...
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Do apparent magnitude (V) and photometric magnitude (specifically in optical V band between 500-600 nm) have a similar scale to stellar magnitude?

I'm graphing a table and I used the magnitude values from this database https://vizier.u-strasbg.fr/viz-bin/VizieR-4 (It's the V/19/data catalog for "Masses and Ages of stars in 68 open clusters&...
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If an object 1 billion light years away emits light, does it take more than 1 billion years to reach us because of the expansion of the universe?

From page 7 of the recent (September 26, 2020) edition of Science News Magazine: Detected May 21, 2019, the gravitational waves came from a source about 17 billion light-years from Earth, making this ...
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1answer
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What is a UBV source for stars or stellar objects?

I was using the V19(VizieR) catalog and the main identifier of the stars was in terms of an 'ID' number which they mentioned as being 'Star number in UBV source' I have no idea what that means. How ...
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Particle horizon distance as $t \to \infty$

So I know that the particle horizon distance in our universe is about 46 billion light years. I also know that as $t \to \infty$, the particle horizon distance approaches a finite value. What is this ...
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How far away are objects whose light will never reach us, because of the expansion of the universe?

I thought I had read this question on Stack Exchange before, but now I cannot find it... In fact, I thought I had posted this question before, somewhere, on Stack Exchange... I believe the answer ...
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Why can't we point the centre of the universe from inflation graph we see?

This question is not about whether it makes sense to have a centre of the universe or not instead, it's about the inflation graph we used to see while describing the Big Bang. From the inflation graph ...
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Units of angle in comoving transverse distance calculation

For two objects at a redshift $z$, I know that the proper distance between them is given by $\delta \theta d_M(z)$, where $d_M(z)$ is the transverse comoving distance. What are the units of the angle ...
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Are the most distant known objects in the universe more than 14 billion light years away? [duplicate]

When I hear about the most distant objects in the universe, such as the recently discovered galaxy GN-z11, their distances are usually stated to be a little under 14 billion light years away.... But, ...
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Are black holes considered part of baryonic content of universe?

Are black holes (like those detected by LIGO, for instance) part of baryonic content of universe or part of dark matter? Why?
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Radio loud AGN restarting as Radio Quiet AGN?

Is it possible for a Radio-loud object (eg., an FRI galaxy) to evolve into a Radio-quiet (eg., a Seyfert) source? Also, can this happen via AGN restart? That is, have there been instances where the ...
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116 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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43 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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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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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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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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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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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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155 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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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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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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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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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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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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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
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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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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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