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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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what are possible explanations for the larger scale anomaly in Planck satellite's data?

In the Planck satellite's image of the Cosmic Microwave Background (CMB), there have been anomalies, the most important of which is a deficit in the signal at large angular scales on the sky, which is ...
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How to Interpret WMAP's statement they have “nailed down the curvature of space to within 0.4% of ”flat“ Euclidean””

I find myself slightly frustrated when I'm reading a NASA page and they claim they have “nailed down the curvature of space to within 0.4% of "flat" Euclidean” 0.4% of what? Mathematically this ...
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Where was the farthest object 13 billion years ago?

Asumme that we see a shape of the gallaxy which reached to us in 13 billion LYs. I understood that the object right now maybe in different shape and is >40 billon LYs away. But, as the universe is ...
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Particle creation, energy density and the Compton wavelength

The Compton wavelength of a particle is given by $$\lambda=\frac{h}{mc}.$$ One can construct an expression for the energy density $\rho$ of a particle of mass $m$ given by $$\rho = \frac{mc^2}{\lambda^...
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How is the horizon length related to the power law spectrum of density fluctuations

For a power spectrum density fluctuations $P(k) \propto k^n$, I need to find the scaling (with respect to $a$) of the horizon wavenumber $\frac{2\pi}{\chi_H}$ in a matter dominated universe in terms ...
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General question about dimensionality of the universe [duplicate]

I am a high school math teacher with some basic physic/lay person knowledge. Each year after the AP test, I show the movies Flatland and Sphereland to my calculus classes and talk about the ...
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Is it possible to survive the destruction of a planet (earth)? [on hold]

I recently watched Star Trek VI: The Undiscovered Country again. The main story evolves around the destruction of a moon of the klingon empire which is their main ...
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Could the theory of everything be an algorithm? [on hold]

I've been thinking about the theory of everything. Could the theory of everything to explain how the universe work be a computer algorithm? Is there any way to test whether the universe is a ...
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BAO : Relation between redshift, Hubble constant and radial

From this link https://en.wikipedia.org/wiki/Baryon_acoustic_oscillations#Measured_observables_of_dark_energy , I can't get this relation : $$c\Delta z = H(z)\Delta \chi\quad\quad(1)$$ with $z$ ...
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Does $\Lambda$CDM include inflation?

$\Lambda$CDM obviously includes dark energy (the $\Lambda$) as well as cold dark matter (CDM). Does it also include cosmic inflation? I've not seen a clear answer to this. Wikipedia's article on ...
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Interpreting Kruskal for cosmological spacetime

I'm dealing with a metric of the form: $$ds^2 = -f(r)dt^2 + a(t)(1/f(r)dr^2 + a(t)r^2d\Omega^2)$$ Where $f$ is just $f = 1 - 2m/r$. The first thing I did was to introduce a conformal time coordinate $...
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Has the pilot-wave model been used to make cosmological predictions?

In Roman Sverdlov’s paper Incorporation particle creation and annihilation into Bohm's Pilot Wave model1 he says: Finally, we have pointed out that the modification of gravity had a side benefit: ...
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What was the informational content of the universe at the Planck time?

If the energy of $1 \, \mathrm{bit}$ of information is $k_{\small{\text{B}}}T \ln{2}$, then the energy of that bit increases with the temperature of the system. When I try to calculate how much energy ...
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What's wrong with the Big Spin Model?

A Dr. Serkan Zorba has a paper on arxiv in which he considers, what if the universe is actually slowly rotating? This gives rise to centrifugal and Coriolis forces on a galactic scale that seem to ...
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Stars with different Speed of light [closed]

Is the speed of light changes for each star depending upon the diameter or the speed is constant irrespective of the diameter in vacuum
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Why are there peaks in the CMB power spectrum?

The CMB power spectrum consists of several peaks of different heights existing at specific angular scales of the temperature fluctuations. Questions What is the significance of these peaks? ...
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Do we have enough time for understanding space? [closed]

May be there more big bangs out there... while studding about space i some times think about a bacteria living on my body that may have a life span of 5 seconds if it has logical understating even ...
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Can somebody tell me what was the size of the universe and the expansion rate at the big bang? [duplicate]

I want to know the exact numbers and preferably the calculations that gives the size of the universe at the Big Bang and the initial rate of expansion at the moment of big bang. Does somebody know ...
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How to decide step size(sigma value) in MCMC routine related Cosmological Parameter estimation?

I am running a modified Lambda-CDM in MCMC routine using Montepython and CLASS code, with aim to extract parameter values from Planck data. In addition to the original 10-15 parameters for Planck ...
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A naive question about the Cosmic Microwave Background

The figure from Wikipedia describes the CMB power spectrum. I have a very naive question about this plot. Question From the horizontal axis above seems that the angular scale is plotted along the $...
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1answer
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If the universe is a $3$-torus, must it be longer in some direction?

So in a finite universe, I've read one possible topology for a flat universe is a 3 torus. On a 2 torus, it's obviously longer in one direction than the other. Would the same hold true for the ...
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Cosmic scale factor $a(t)$ in a general frame of reference

In considering an FLRW type universe, the scale factor $a$ is generally indicated as being strictly a function of time $a(t)$. Isn't this only true for a comoving frame? In some other reference ...
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Contracted Bianchi Identity for FLRW Metric

I am trying to verify that the Contracted Bianchi Identity $\nabla_\mu G^{\mu\nu}=0$ holds for the Friedmann-Lemaitre-Robertson-Walker (FLRW) metric, $$g_{00}=-1,g_{i0}=0,g_{ij}=a^2(t)\delta_{ij}$$ ...
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When/ how were/are quarks formed?

We know that all particles can eventually be converted into energy. We know also that electrons were and still are formed by pair production from a 1MeV photon. Do physicists have yet any idea how ...
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Does Coleman-de Luccia instanton approach a Hawking-Moss instanton?

Suppose a Coleman-de Luccia instanton terminates in the basin with the true vacuum between the top of the potential barrier and the field value with potential energy equal to the potential energy of ...
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Universal Entropy [duplicate]

I’m a first-time poster... Could entropy possibly be the reason the universe is expanding and even why its expansion is accelerating? If space is not surrounded by anything at all, it would seem to ...
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1answer
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What does the matter density in density parameter mean?

When we write $$\Omega_m = \frac{\rho_M}{\rho_{\mathrm{crit,o}}}$$ what density does $\rho_M$ refer to? As in, suppose our universe was made with just hydrogen. Then would $\rho_M$ mean just the ...
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Functional freedom in cosmology. Why does it matter?

Roger Penrose's book "Fashion, Faith and Fantasy in the New Physics of the Universe" is a fascinating (and challenging) read, but while I understand his mathematical points about how additional ...
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Matter Anti-matter Imbalance

If both matter and antimatter could still 'pouring out', why do we assume that there is an overall Matter Anti-matter Imbalance? Could not antimatter possibly come out during a different time as ...
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1answer
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An alternative version of Olbers' Paradox

I've recently stumbled upon an alternative version of showing Olbers' Paradox analytically, namely the following problem from Arfken and Weber's Mathematical Methods for Physicists: Assume a static ...
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Results of WMAP mission

The NASA WMAP mission mapped the cosmic microwave background (CMB), the the residual high temp photons from the condensed space 380000 years after big bang that expanded into the much high volume we ...
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Mass equivalent of CMB [closed]

If the 2°K CMB is the portion of big bang energy that didn't get turned into bayonic/dark matter or dark energy (as it now stands) at 380,000 years post bang, how big is the CMB energy compared to ...
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Travelling at lights speed of light as universe expanding

I have read that the object, the light of which is reached us in 13.8 billion years is actually about 46 billion light years away from us now, due to the expanding Universe. Lets assume that we ...
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How age of universe or galaxy estimated? [duplicate]

How scientist found age of universe as 13.8 billion years and age of Milky Way as 13.51 Billion years? Is there any proof?
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What is the definition of gravitational horizon $R_h$ in the $R_h=ct $ universe?

Melia's $R_h=ct $ universe proposal suggests that our gravitational horizon $R_h(t)$ should be equal to $ct$ for all cosmic time $t$, not just its present value $t_0$: "several unpalatable ...
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Why is the observable universe 90 billion light years in diametre? [duplicate]

The question is fairly simple, we know that the universe has been expanding for 13.8 billion years, so how did they determine the size of the universe that we can observe, and why can we not peek ...
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Derivation of relativistic pressure

As you can find in many cosmology textbooks, the relativistic pressure in quantum statistical mechanics can be witten as below: $$p=g \int \frac{d^3P}{(2 \pi \hbar)^3} \frac{c^2 |\mathbf{P}|^2}{3E(\...
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Was there a Quantum Universe before the Big Bang? [duplicate]

I have seen some similar questions but never exactly this, is it possible that the Quantum realm existed before what we see as the Big Bang? Was the big bang just the beginning of 3 Dimensional space ...
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How can the effects of dark energy be distinguished from the effects of the acceleration of the universe from the big bang?

This is a conceptual question and has probably been asked before in many guises and forms. Mine has a little twist to it. I realize the Hubble Law tells us the farther back in time and space we look ...
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Einstein's static universe concept

This question is I think related to Einstein's static universe. I was reading an article on it that I am quoting here, and I have some very simple questions about it. My questions are interspersed. ...
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Why can we see the cosmic microwave background radiation?

This radiation (CMBR) is said to have its origin at the surface of last scattering that exposed itself when the big bang universe had expanded for less than a million years. In order to see radiation ...
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Why isn't an infinite, flat, nonexpanding universe filled with a uniform matter distribution a solution to Einstein's equation?

In Newtonian gravity, an infinite volume filled with a uniform distribution of mass would be in perfect equilibrium. At every point, the gravitational forces contributed by masses in one direction ...
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Why does inflation end when the potential becomes too steep?

Inflation ends when the potential becomes too steep. Is it because kinetic energy increase when the potential becomes steeper?
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How the matter and radiation densities of universe can be calculated? [closed]

Barbara Ryden gave somehow detailed and intuitive calculation in Article 5.1 of Introduction to Cosmology but skipped detail on some values. I don't know, it's good or not but I'm adding snaps from ...
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Does almost constant energy density only require that $ \omega \approx-1 $?

From continuity equation \begin{equation} \dot{\rho}=-3H(p+\rho) \end{equation} $ p+\rho\approx0 $ will give $ \dot{\rho} \approx 0 $ which means almost constant energy density From equation of ...
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Conformal diagram of FRW metric

what transformation is used in making the conformal diagram of FRW? I've tried using the $u=t-r$ $v=t-r$ but I dont seem to be getting anywhere. actually, is there a "recipe" for making diagrams ...
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How is it possible to infer the time it takes for photons to reach the earth?

The photons emitted from a long, long time ago in a galaxy far, far away arrive at our planet several million or even billion years after the light was first created. So, how do we know, when we ...
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How can we linearly extrapolate current expansion of universe to imply that it all started from a point?

We know that the universe is currently expanding. But how does that imply that it all started from an infinitesimally small point? Its like given the stock price of Apple is increasing, it doesn't ...
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Is the universe ~14 billion years old or that's the farthest photons which reached Earth?

I know that the universe: It's around 13.772 billion years old It expands But it's not clear to me if this is not merely the age of the farthest known photons which reached Earth.
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Is cosmological redshift at a given time has a constant value?

On one hand, people say that various cosmic events occurred at various redshift values. For example, recombination happened at $z=1100$. This would imply that at any given point in time, in the ...