Questions tagged [cosmology]

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

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following several unknown phenomena detected at the edge of the solar system, is there a 5th force which acts at the edge of the solar system?

following several unknown phenomena detected as the acceleration of Oumuamua and electrons and strange data sent by traveler 1 at the edge of the solar system, is there a 5th force which acts at the ...
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why the redshift should be very small( z<1) to apply h^-1 parameter? [closed]

can anyone give a brief answer to my question either proving mathematically or by logic I can prove when comparing to doppler effect but don't know how to prove when comparing with the parameter or ...
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Why do most astrophysicists believe the cosmic microwave background (CMB) provides the best evidence for dark matter? What, exactly, IS that evidence?

I frequently read that the cosmic microwave background contains the best overall evidence for the existence of dark matter, and conversely against alternative gravity theories like MOND. However, I ...
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How would a balloon structure of a universe look if we collected all mass together in a small region?

Imagine a universe with an inflating balloon structure of spacetime. Matter is distributed uniformly. Now somehow we manage to collect all matter together within a small volume. What would happen to ...
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If the universe is a closed spacetime structure, can we assign a center of mass to it?

If we envision the universe like a balloon but in 3D, is it possible, in general, to assign a of global center of mass?
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Is there a cosmological model of the evolution of vacuum motion in our universe over time? [closed]

Is there a cosmological model of the evolution of vacuum motion in our universe over time? Does this model starting from a non-vacuum constitute an advantage compared to the cosmological model of ...
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Plotting galaxies in 3-D

Suppose I have a catalogue of galaxies in a .dat file with each row representing information about a galaxy: right ascension of the galaxy, declination of the ...
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2 votes
1 answer
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Why is the CMB's redshift so high?

It is pretty well-established that the CMB was originally emitted around 380,000 years after the Big Bang, at a redshift of ~1100. The most distant known object is HD1, the light from which was ...
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Will the upcoming experiment PICO measure kSZ temperature anisotropy? [closed]

I know for sure PICO will be measuring polarization anisotropies with high fidelity. In addition, the PICO science paper shows that it will make full-sky Compton-y maps but the plots are mostly ...
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Interpretation of Hubble constant in SI units

The standard interpretation of Hubble constant $\approx 70~\text{km/s/Mpc}$ means that each mega-parsec of distance adds $70~\text{km/s}$ to a galaxy recession velocity from us (or to a space ...
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Angle to the line of sight

I don't understand this angle used in galaxy clustering. There is the line of sight and this angle to the line of sight which is not clear to me. Here is a transcript of a description from 1705.05442: ...
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By dint of some 'reverse engineering' I have produced the equation below. Does anyone recognise it? Does it come from general relativity?

The equation is $$a = \frac{\sqrt{2} \cdot c^2}{k}$$ where a is the acceleration due to gravity, c is the speed of light and k is the radius of curvature of the geodesic. The equation is dimensionally ...
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Expansion rate of an infinite universe at the Big Bang

If the universe is infinitely large, then any two arbitrarily distant points must have been arbitrarily close together at some earlier point in time. Doesn't that mean that the expansion rate of the ...
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How are CMB frame redshifts of galaxies corrected for coherent flows?

Say, if I downloaded the CMB frame redshifts of some galaxies from NED database, then what is the procedure to correct it for coherent flows? I'm using the SNooPy(snpy) Python package to analyze some ...
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If baryogenisis had not occurred how would the universe progress and how would it differ from CCC's final stage

If baryogenisis was an error and there were no quarks and leptons remaining, can we assume that inflation would have still occurred and continued without matter. Would other particles have survived in ...
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Stellar connectivity for different cosmological redshifts

I'm creating models of stellar connectivity, for a range of cosmological redshifts (z=9, z=7, z=5, z=3, z=1, z=0, z=0.5). For that I'm doing the following steps: Sampling simulation sub-halos for ...
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1 answer
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Time & The Big Bang [duplicate]

What are the arguments against a perpetual universe, i.e. one that expands from a singularity, like we observe with ours, then suffers from terminal entropy, collapses back upon itself before reaching ...
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How far away from us was Earendel star when it emitted the light that is just now reaching us?

From what I've read, the look back time is about 12.9 billion light years, and the current distance to the Earendel star is approximately 28 billion light years... How close to us was it when it ...
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Does the Cosmic microwave background (CMB) have an amplitude? Does it vary, like the 'temperature' (wavel./freq.) and the polarization?

Somehow, I have never read about this or thought about, until now... Does the number of photons from the CMB hitting us from all directions vary at all?
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Strange plot in Max Tegmark book, Our Mathematical Universe

In Max Tegmark's book, Our Mathematical Universe, we can find (in chapter 5, figure 5.3) the following (horrible and poor quality) plot that is supposed to highlight the extreme sensitivity of the ...
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What is the 'TE correlation' in the CMB, or 'temperature-E-mode' correlation, aka 'temperature-polarization' correlation?

There are several places on the web that mention this, a couple in some technical detail, but I cannot wrap my head around what exactly it means...
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4 answers
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For what aspects of cosmology research can a software engineer contribute?

I'm a software engineer and I would love to learn more about cosmology by participating in this field. I'd like to change my career path closer to it and further from web programming. But I don't know ...
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8 votes
4 answers
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Does the universe have a celestial north?

You are away from the Earth in one part of the universe and looking at a galaxy in another part of the universe, then which way is up? Does the universe (or near universe) have a celestial north? If ...
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3 votes
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Reasons for extremely high number density of galaxies at low redshift

I am currently analyzing the SDSS data especially in the extremely low redshift area, in order to measure the cosmological statistics. What I mean by extremely low is z<0.1. One peculiarity that I ...
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How do I create a comoving quantity using YT?

I can assign a unit to a scalar using: x = YTQuantity(scalar, 'kpc') but how would I specify that the unit should be kpccm? Perhaps it doesn't even make sense to do so? Something as simple as this is ...
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Is there physical evidence to distinguish between the expansion of space and an anthropocentric universe?

When we look in all directions, we see distant objects red-shifted, with the size of the red-shift correlated with the distance from us. As I understand it, the consensus among cosmologists is that ...
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How is observable matter distributed in the universe?

The observable matter in the universe is distributed and arranged into various structures, including black holes, stars, nebulae, and the much more diffuse regions of the inter-stellar and inter-...
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14 votes
1 answer
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What happened to the reemitted photons during recombination?

To my understanding, the Cosmic Microwave Background Radiation (CMBR) is light released during the recombination epoch where the formation of neutral hydrogen atoms allowed for a sudden drop in the ...
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How does the gravitational interaction between dark a matter bulge and an ordinary matter disk affects stability over time?

From Cosmology it is known that a spherical bulge of Dark Matter, that only interacts gravitationally, surrounds the nucleus of each galaxy, that has a spinning disc component of ordinary matter. How (...
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3 votes
1 answer
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Doppler Redshift vs. Cosmological Redshift ... or Both?

There are several existing threads on the difference between cosmological and Doppler redshifts. However, I don't see that any of them answered the question below. @pela gave the following example: &...
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What would have been the case if the Hubble diagram was linear throughout?

Redshifts are converted into velocities using v = cz; this is a good approximation for low redshifts over which the Hubble diagram is linear. Since the Hubble diagram deviates from linearity at high ...
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3 votes
3 answers
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Why does the gas cloud collapse in regions of high density?

Stars form when gas cloud collapse under gravity, becoming hot and subsequently initiating nuclear fusion. I have read that the collapse is triggered by density fluctuations, where regions of high ...
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1 answer
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Intergalactic Lyman-alpha absorption for high redshift quasars (Gunn-Peterson effect)

This is a follow up to a recent question on SE asking about the apparent suppression of radiation shortward of the (red-shifted) Ly-$\alpha$ line of a quasar at redshift $z=6.53$. The general ...
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6 votes
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Can we consider the Universe to be some kind of 3-sphere?

This is probably a naive question. I'm learning a bit of cosmology and I've recently covered the so called angular size-redshift relation, which states that in an expanding Universe the angular size ...
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What does dimensionless Jerk=1 mean?

Looking to see if the cosmic acceleration is increasing, steady or decreasing
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What exactly is the estimated distance to a far-off object when they say '50 Mpc/h' or '50 Mpc h^-1'? Is it less than 50 Mpc? Do you ÷ by 67 or 74?

Several recent arxiv.org papers I read mention distances to very distant objects in Mpc (megaparsecs) divided by Hubble's 'constant'.... Does that mean we should divide the Mpc or Gpc (gigaparsecs) by ...
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4 votes
2 answers
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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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2 votes
1 answer
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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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8 votes
1 answer
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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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1 vote
2 answers
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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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1 answer
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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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2 votes
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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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9 votes
1 answer
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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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1 vote
0 answers
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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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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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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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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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1 vote
0 answers
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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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5 votes
1 answer
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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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