Questions tagged [cosmology]

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

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2answers
99 views

How can it be justified that every discovery at z > 1 is an indication of "slowing down" or past deceleration?

Type Ia supernovae discoveries at high redshifts (z > 1) support past deceleration (Riess et al. 2001). This past deceleration has also been confirmed using quasars (z ≈ 6) (Risaliti and Lusso 2015)...
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1answer
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Is our Milky Way galaxy (and its neighbors?) moving through the universe at 370 km/s (~828,000 mph) or 370 miles/sec (~1,332,000 mph)? [duplicate]

I swear that exactly half of the many, many sites I found say that our Galaxy and its immediate neighbors are moving towards the Leo Constellation (I believe) and, ultimately, supposedly, the 'Great ...
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1answer
1k views

Why was the Cosmic Microwave Background (CMB) released at a blackbody temperature of 3000K rather than 30,000K?

If the ionization (or reionization, or Recombination) energy of atomic hydrogen is 13.6 EV, which corresponds to a black body temperature of 30,000K, why did the CMB not begin to appear then? Why did ...
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2answers
101 views

Are there negative masses that attract each other but repel positive masses? [closed]

Is there law/rule in physics that precludes the existence of a negative mass, wherein similar negative masses attract but positive and negative masses repel? To keep this on-topic in Astronomy SE: Has ...
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0answers
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Do Einstein's ten field equations use 20 or 40 variables? (2 or 4 for each tensor equation?)

One site I came across says Einstein's 10 Field Equations use 20 variables, while another said 40. There are four variables in spacetime - three for space and one for time, right? But there are two ...
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0answers
62 views

What new fundamental knowledge could a very, very large optical telescope bring? [closed]

Very large baseline radio telescopes bring many mysteries of the cosmos to us. We build them because they are feasible. Say, in the future, we could build very large optical telescopes in deep space, ...
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1answer
868 views

Why isn't the CMB blurred by the blackbodies emitted in the time after atoms first formed?

I am confused about why the light released from the moment when gas first formed was so dominant in comparison to the light released afterwards. Why isn't the CMB in interference with a series of ...
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0answers
110 views

How did the Universe climb out of its own Big Bang black hole?

The Big Bang started as a singularity. That means small. All the matter in the universe was in a volume smaller than its own Schwarzchild radius. The universe was inside a black hole. At present, the ...
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0answers
52 views

Are there galaxy clusters or dark matter halos in the void?

The void usually refers to the region of the Universe where the overdensity $\delta(x)$ is less than zero. What I'm curious about is whether there are virialized objects, like galaxy clusters or dark ...
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1answer
116 views

If the ratio of actual total mass to critical mass (Ω) in the universe is only about 0.3, why do cosmologists believe it is flat, as far as we see?

What about astronomical observations makes scientists believe our universe is flat, at least as far as they can tell? Despite the critical Friedmann density being less than one, plus the existence of ...
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1answer
24 views

Calculating comoving volume of 2dF survey

Say I want to calculate the comoving volume in the 2dFGRS survey: I'm using $H_{0} = 70 km s^{-1} Mpc^{-1}$ and the default values for the matter and vacuum density parameters in this calculator (...
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How can scientists deduce the number of types of neutrinos, or 'effective number', from Planck satellite data?

Is it related to the way they deduce the Hubble constant from Planck data? Would more types of oscillating and mixing neutrinos mean faster or slower expansion of the universe? Would a fourth mass ...
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1answer
77 views

Minimum size of closed universe that satisfies ΩK < 0.005

According to this Wikipedia article: https://en.wikipedia.org/wiki/Shape_of_the_universe Results of the Planck mission released in 2015 show the cosmological curvature parameter, ΩK, to be 0.000±0....
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How sensitive are early universe abundance calculations to the neutron half-life?

Phys.org's Physicists announce the world's most precise measurement of neutron lifetime discusses the accepted for publication Phys. Rev. Letter Improved neutron lifetime measurement with UCNτ (links ...
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Cosmology context plotting : Covariance matrix - Script to plot Omega_m vs Omega_Lambda diagram in Python

I have covariance matrices and Markov chains coming from the SCP (Supernova Cosmology Project) and I would like to plot all these data in the particular diagram Omega_Lambda vs Omega_m or w vs Omega_m....
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1answer
43 views

What's different of power spectrum and correlation function of BAO?

(source) There is the sentence "the harmonic sequence in the power spectrum correspond to a single peak in the correlation function and that the damping envelope corresponds to the broadening of ...
3
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1answer
88 views

Is the sound horizon 500 thousand light years or 500 million light years?

At 06:30 in the nice video Sound Waves from the Beginning of Time about the baryon acoustic oscillations (BAO), the speaker says the sound horizon is about $500$ ...
2
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1answer
63 views

Observability of proper distance

Hubble's law at current time is as follows. $$v=H(t_0)r$$ But if you look at the explanation on Wikipedia: Strictly speaking, neither $v$ nor $D$ in the formula are directly observable, because they ...
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2answers
77 views

The Lambda-CDM-model: when will the Age of the universe catch up with the Hubble time?

Basic assumptions: A) Within the Lambda-CDM-model we have two separate time measures: the Age of the universe - today roughly 13.8 GY, and the Hubble time - today roughly 14.4 GY B) Very roughly, ...
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1answer
427 views

Why would a quantity like the 'Hubble contrast' be squared, then have its square root taken?

From Sabine Hossenfelder's recent video, New Evidence AGAINST Standard Cosmology: And her source.... Figure 2. The variation with increasing void radius of the variance of the Hubble parameter, the ...
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1answer
72 views

What does lowercase 'beta' mean in this graph of radio-emitting galaxies?

Correlation between the mass of the galaxies (X-axis) and the difference of their radio emissions at different radio frequencies (Y-axis). Each symbol represents an individual galaxy. The image of an ...
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0answers
24 views

What is Euclidean fluence scaling in the context of bounding constraints on FRB all-sky rates? Would non-Euclidean scaling give a different answer?

The recent Chromatic periodic activity down to 120 MHz in a Fast Radio Burst contains two paragraphs in which X's are given "assuming an Euclidean fluence scaling". To me, a non-astronomer, ...
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50 views

Are there ideas about Multiverse in cosmology that are independent of string theory?

It is known that string theory allows for many kinds of universes within it's Landscape which give rise to the concept of the multiverse. But are the concepts/ideas about multiverse in cosmology which ...
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Why does the 'Big Bang Nucleosynthesis' theory require that neutrinos, or at least sterile neutrinos, have a LARGE mass rather than a tiny one?

From Wikipedia, Sterile neutrino Particles that possess the quantum numbers of sterile neutrinos and masses great enough such that they do not interfere with the current theory of Big Bang ...
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66 views

Do some physicists believe that the universe is a three-dimensional space wrapped on a four-dimension doughnut?

The astrophysicists believe if this is true, the universe would be finite. The entire cosmos may be only three or four times larger than the limits of the observable universe, which is about 45 ...
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2answers
113 views

How does the hypothesis of the "inconstant Hubble constant" solve the current crisis in cosmology?

It was published in a paper more or less like two months ago. I'd like to know also if more accurate measurements are necessary to close the gap between the model of the universe and the data reported....
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4answers
291 views

Why do we see a cosmological redshift at all if space is not expanded in our solar system?

It is generally accepted that the large scale redshift of galaxies (as given by the Hubble law) is due to the expansion of the universe. According to this theory, the fabric of space itself stretches ...
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1answer
71 views

How can we know the density of particles in interstellar or intergalactic space and could that density form dark matter?

It is assumed that the density of particles in the empty space between stars or galaxies is extremely low. But how do we know this density? The space surrounding the Sun is contaminated by cosmic ...
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2answers
4k views

Why do gravitational waves travel "only" at the speed of light but the gravitational scalar potential is instantaneous?

To my old Space SE question Besides retarded gravitation, anything else to worry about when calculating MU69's orbit from scratch? @DavidHammen's excellent answer replies that one should not add a ...
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1answer
150 views

Is it "nonsense to even talk about" objects outside the observable universe not having gravitational influence on us? (finite speed of gravity)

In this supplemental answer to Is the zero gravity experienced in ISS the “artificial” kind? in Space Exploration SE I said: Gravity moves at the speed of light so nothing outside out observable ...
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Formula to calculate round-trip travel time based on current distance to far-away galaxy

Cross-posted from Physics Stack Exchange - original The question was closed as HW-like there. The Premise You travel at a constant velocity towards a far-away galaxy. You then travel back at the same ...
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3answers
299 views

Could it be that Universe is rotating while expanding? [duplicate]

I have read that the measurements of the cosmic microwave background indicate that the Universe has a flat geometry on large scales. Could one assume then that the Universe is acceleratingly rotating ...
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0answers
23 views

How to locate helpful and instructive books and resources about numerical methods in cosmology?

I am interested in the hydrodynamical and N-body simulations, and I want to use simulation results to examine the theoretical models. Hence I am searching for some helpful and instructive books and ...
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4answers
2k views

How can a black hole have a charge, or be charged?

So-called 'hairless' black holes (no-hair theory, or theorem?) , which is what real black holes are, can be described by just three characteristics: Mass, spin, and charge. It is easy enough to ...
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0answers
50 views

What are the one-dimensional observables that can reveal the matter clustering properties of the Universe?

As we all know, the distribution of galaxies, which depend on the spatial coordinates, can help us understand the large-scale structure of the Universe. We can measure the distribution of galaxies ...
3
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1answer
43 views

Dark Matter Density Parameter Variation

The definition of the dark energy density parameter is $$\Omega_{\Lambda} = \frac{\epsilon_{\Lambda}}{\epsilon_c}$$ where $\epsilon$ is the energy density, $\Lambda$ subscripts represents dark energy ...
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1answer
183 views

If we watched extremely red-shifted galaxies near the edge of the observable universe for a very long time, how would they change? Would more appear?

I have understanding sphere eversion as #1 on my bucket list (if I ever get a round tuit) but understanding metric expansion seems to be a rapidly receding possibility :-) Question: Suppose it takes ...
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0answers
34 views

How to find the scale factor on given time in the future if initial distance between source and destination is known

Assume the received redshift $z$ of a photon is known. We then can calculate the initial distance (when the received photon was emitted), the light Travel Distance, the current Distance (proper ...
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0answers
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Questions re galaxies clusters and galaxies mergers within them

An introduction (containing the info I gathered from several Wikipedia articles). A galaxy cluster, or cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of ...
3
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1answer
66 views

Scalar field displacement from the minimum of the potential gives rise to particles/dark matter, why?

The paper Lyman-alpha Constraints on Ultralight Scalar Dark Matter by Kobayashi et al. says, at the beginning of Section 3.1: A light scalar field stays frozen at its initial field value in the early ...
2
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1answer
34 views

What do "overdense" and "underdense" mean in cosmology?

“Overdense" and "underdense" often appear in various documents to refer to high density and low density respectively. And I know that "overdensity" is defined as $$ \delta(x)=\...
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0answers
32 views

Explain as simply as possible how the Sunyaev-Zeldovich (SZ) effect is used to estimate the Hubble constant

The Sunyaev–Zeldovich effect (SZ effect) is useful in determining the Hubble constant because it is independent of the cosmic distance ladder. This effect occurs when CMB (cosmic microwave background) ...
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1answer
110 views

Energy conservation of the universe

I have tried to derive an equation for the total energy of the universe. I have found that, $$E(t)= \delta\dot a(t)^2a(t)\Omega(t)$$ Where $\delta$ is just a positive constant, a(t) is the scale ...
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1answer
243 views

Comoving Volume Calculation

Suppose I have data from an astronomical survey at redshifts in the range $z = [2,3]$. Suppose that, on average in this range, the data covers an area on the sky of $A=1$ $\mathrm{Mpc}^2$. How would I ...
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0answers
50 views

How did pressure evolve in the (early) universe?

I am trying to derive how the cosmological pressure $p(t)$ evolved over time in the universe, especially in the radiation and matter dominated epochs. There are some very nice explanations how $H(t)$ ...
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1answer
72 views

Would it be a good idea to dockerize softwares in order to conduct hands-on tutorial sessions? [closed]

I am going to organize a tutorial in which I intend to train students on semi numerical simulations for reionization. Is it a good idea to supply use Docker to supply the codes? What do you think ...
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0answers
30 views

Looking for analytical expression of Cosmic Variance - Poisson distribution?

I have an expression of Matter Angular power spectrum which can be computed numerically by a simple rectangular integration method (see below). I make appear in this expression the spectroscopic ...
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2answers
85 views

Does black hole singularity really exists with respective to relative frame of reference? [duplicate]

How can one describe this phenomena? What are the factors that we should consider for solving such a problem?
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0answers
99 views

Where does the energy of the decaying CMB go?

I would expect that energy of photons in the Cosmic ray Microwave Background gets less and less because their wavelengths are stretched due to the expansion of space. How can this be possible? Does it ...
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3answers
218 views

How is it possible that a universe with mass does not have curvature?

I may know the answer to my question. Mass must curve the universe but the pressure due to energy density may curve the universe in the opposite direction, leading to a flat universe, if they exactly ...

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