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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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How do we quantify and measure the net entropy of today's universe?

We know that the Universe evolved from a very low entropy state in the early universe to very high entropy state of today's universe. What is the quantitative definition of the net entropy of the ...
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1answer
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$W$, $Z$ bozons and fermions gained their masses WHEN?

I have read these questions: How does the Higgs mechanism work? How does the Higgs boson give mass to other elementary particles like electrons? What is the difference between the Higgs Boson ...
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Meaning of “observable scales” in cosmology

This is something everyone reads about in cosmology texts or research in cosmic inflation- there is a cosmological pivot scale $k_{\star}=0.05$ or $0.002\;\text{Mpc}^{-1}$ at which relevant parameters ...
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4answers
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Is the fine-structure constant related to the size of the observable universe?

The fine-structure constant $\alpha \approx 1/137$. In Planck units, this is also the charge of the electron squared, $e^2 = \alpha$ ($e \approx 0.085$). In Planck units the size of the observable ...
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How Can the Universe Be Bigger Than It is Old? [duplicate]

My question is this: If we can look back to roughly 500m years after the big bang and have estimated the age of the universe at 13.7B years, why is the galaxy we are looking at not a part of the non-...
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Express polarization moments in terms of temperature quadrupoles

I am trying to compute the polarisation moments in the tight coupling limit which is an exercise from Dodelson's Modern cosmology where the evolution equation is given by $$\dot{\Theta}_p+ik\mu\...
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Critical density of positively/ negatively curved space

From the friedmann equation $$1=\frac{\rho(t)}{\rho_c(t)}-\frac{k}{\dot a^2}$$,if $$\rho=\rho_c$$then either k=0 with $\dot a\ne0$ which is expansion/contraction of flat universe. Or for $$k=+-1 ,\...
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Why did recombination make the universe transparent?

It is commonly said that after the universe cooled enough for ionized Hydrogen to settle down into neutral Hydrogen, i.e. recombination, the universe became transparent. A reason I have heard for this ...
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1answer
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Entropy of the big bang

At the moment of big bang, all the matter was in perfect order, that is entropy 0 so what force or disturbance would occur to begin the chaos and the entropy start to increase?
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1answer
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Could gravitational waves give an illusion of an expanding universe? [on hold]

‪Could the effect of “expanding” gravity (gravitational waves spreading at the speed of light) as it travels through space (and therefore becomes less local) red-shift all EM waves and thus give us ...
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What is wrong about the following reasoning concerning hypersphere cosmology? [on hold]

Light traveling through a hypersphere would constantly curve while appearing to go straight in three dimensions. This would mean light gets stretched over greater distances than those we measure based ...
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1answer
41 views

How do we know the quantity of dark matter in the universe?

I have been reading on dark matter and it is given specific numbers of its quantity, i.e, ~27%, in the universe. How we come up with that number as we have no strong opinion what it is made of? We ...
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$\rho(t)\gt or \lt \rho_{critical}(t)$ depends upon $k$ for expansion or contraction in cosmology?

From friedmann equation $$1=\frac{\rho(t)}{\rho_c(t)}-\frac{k}{a^2H^2},$$$$\dot a(t)=+-\sqrt\frac{k}{\frac{\rho(t)}{\rho_c(t)}-1}$$ for $k\gt0$,$$\rho(t)\gt\rho_c(t)$$ and for $k\lt0$,$$\rho(t)\lt\...
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Hamiltonian ordering ambiguity in quantum cosmology/gravity

I am trying to study several quantum cosmology models. The standard procedure for quantization consists typically in several steps: People write the theory as an action, or Hamiltonian $H(p^i,q^j)$ ...
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1answer
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Could universe expansion continue after the universe reached thermodynamic equilibrium

Stimulated by reading this question Would time still exist after Heat Death of the Unverse? it seems that a literally eternal expansion is not precluded. This would be in accordance to a real ...
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1answer
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Would time still exist after Heat Death of the Unverse?

From my (honestly limited) knowledge of what the Heat Death of the Universe is, I understand that HDotU is the point when energy levels across the entire Universe reach equillibrium, thus energy doesn'...
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Integral of Bolzmann distribution function for dark matter (modern cosmology)

In Dodelson's textbook modern cosmology ch4.5, I wonder if anyone knows how he did the integration by part over the phasespace on the distribution function of Eq(4.72) and Eq(4.78). Here is part of ...
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1answer
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Plotting the expansion of the Universe Question?

Hi, I'm a first year physics student. Today we were given a picture like this showing space expanding vs time. Now, i have just added onto this picture the path of where light could have come from ...
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Friedmann equations and dependence on each other

I have quick question regarding Friedmann (for FLRW cosmology) and dependence of 00 th component and ii-th component (space-space). Are they equivalent? For example authors of https://arxiv.org/abs/...
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0answers
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In what ways is eternal inflation less certain than standard inflation? [closed]

Eternal inflation is the idea that inflation could be eternal due to the effect of quantum fluctuations of the inflaton field. Why do some cosmologists accept inflation, but consider eternal ...
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1answer
46 views

Traveling to another galaxy impossible for ever?

If galaxies are moving apart from each other faster than the speed of light, does that mean that it's impossible to travel to another galaxy, regardless of what technology get discovered? Because you ...
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0answers
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Choosing a research topic for pursuing PhD in cosmology [closed]

I am about to start my PhD in cosmology and my guide has told me to find problems in the areas of dark energy or general relativity. As a new comer I know that dark energy is somewhat responsible for ...
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Meaning of time and distance in general relativity?

I have just started studying cosmology and I have realised that certain statements (that made perfect sense before I studied GR) no longer make sense: -The universe is 13.2 billion years old. What ...
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1answer
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Why only $\omega=-1$ is taken to explain accelerated expansion of the universe and named dark energy?

From the acceleration equation in cosmology the condition for acceleration comes out if $$\rho+3P\lt0$$,$$P\lt\frac{-1}{3}\rho$$,$$\omega\lt\frac{-1}{3}$$. For $\omega=-1$ it gives a constt. energy ...
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1answer
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Dark energy and dark matter difference

From the rotation curve of spiral galaxy it is found that for (>>r ) the total mass is not concentrated at the centre but varies as ~r for which the rotational velocities of the stars far from the ...
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1answer
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How the cosmic inflation solves the horizon problem (an exact solution)?

I am reading an article, Inflation and CMBR by Charles H. Lineweaver. https://www.mso.anu.edu.au/~charley/papers/canberra.pdf (Page 5/13) He explains the inflation period as the shrinking of the ...
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2answers
53 views

$z$ (redshift) at the inflation start up time?

The $q$ parameter is at present -ve which implies accelerated expansion , hence we can approximate our universe now to be dominated by dark energy since dark energy provides exponential growth $~e^{\...
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0answers
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How are parameters in inflation potential determined

This is a question on how to determine the parameters in a given inflation potential. I have been studying a particular inflation model (https://arxiv.org/abs/1805.09483) which works with a potential ...
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0answers
37 views

Big Bang Hypernova Hypothesis? [duplicate]

Did the Big Bang explosion follow the same dynamics as a hypernova of a super massive star? In a hypernova explosion the implosion at the core gives birth to a black hole. Any stellar material that is ...
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0answers
108 views

Does the observable universe lose mass?

As energy is generated from nuclear fusion in all the stars in the universe, does the (observable) universe lose mass in total? How much loss would that be and what is the effect on us (e.g. is there ...
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Determine Field of View from Given Simulation Box Size

I want to determine the field of view as would be seen by a telescope corresponding to a simulation box size(comoving) of, say 100^3 Mpc^3.As I understand the box size for simulation is a rough ...
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1answer
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Hubble - Cosmological vs Doppler Red Shift

When we learnt about the Hubble Constant at school we were taught that Hubble had observed Red Shift due to the Doppler Effect, and thus the universe (and space time itself?) must be expanding. ...
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Ensemble average VS spatial average for a cosmological density field

I'm studying statistical properties of a cosmological density field and I have some doubts, in the following quote and pictures you can see what my problem is: A fundamental limitation arises in ...
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1answer
103 views

Problems with Twistor theory

Penrose developed a theory called "twistor theory" that tries to describe the universe using twistors. (https://en.wikipedia.org/wiki/Twistor_theory) From what I've read, there are a lot of papers ...
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1answer
28 views

How does the CMB constrain the baryon asymmetry?

The CMB contains information about baryon acoustic oscillations in which baryons (I assume protons and electrons) and photons form a plasma exhibiting sound waves. How is information about the baryon ...
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1answer
38 views

Flux in luminosity distance

I have gone through luminosity distance once again and found out that if $L_s$ is the energy emmited per unit time from the source ,then at any distance d the flux is $F=\frac{L_s}{4πd^2}$,for ...
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1answer
97 views

Why is there equal protons and electrons in the universe?

Matter and anti-matter are asymmetric. Why are protons and electrons symmetric? Would we still have galaxies and stars if there were a strong overall charge?
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1answer
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How would a difference in electron and proton charge be detectable in astronomy?

How would a deviation in neutrality of matter affect astronomy? Obviously this would introduce a lot of repulsive force but how, exactly, would that affect the universe? If the repulsive force was ...
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76 views

Ergodic Hypothesis in cosmology

I'm studying primordial fluctuations of the Universe from a statistical point of view and I'am aware of the following problem: A fundamental limitation arises in cosmology – because there is only ...
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236 views

Does string theory accommodate effects of dark energy?

String theory is arguably, among best candidates for a Theory of Everything. So, because every TOE is an attempt to bind together the big and very small effects and things, a TOE must be able to ...
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1answer
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Antiparticles, CPT and leptogenesis

When people are being careful they'll tell you that antiparticles are the CPT conjugates of particles. You can't say that they are C conjugates or CP because these, while they do reverse the charge, ...
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1answer
57 views

Spacetime diagram of LCDM

I have a question concerning the LCDM spacetime diagram https://i.stack.imgur.com/Uzjtg.png published on the Physics Forum Stack Exchange website Can space expand with unlimited speed? How are the set ...
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41 views

Luminosity distance

If light is emmited from galaxy A at $t_e$ and received at galaxy B at $ t_o.$ The flux at B is $$ (1) -- \phi(t_o)=a^2(t_e)L(t_e)/4πd_L^2$$ $d_L$ being the current distance between A and B In the ...
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Why do we have constraints on the baryon asymmetry of the universe and not the lepton asymmetry?

What is it about baryons that means that CMB observations can determine their asymmetry without also including a charged leptonic component?
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1answer
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Universe inside atom [closed]

It's not my personal theory. Many scientists raised questions about this: https://news.softpedia.com/news/Top-Physicists-Ponder-On-the-Idea-of-Universe-In-An-Atom-93428.shtml https://en.wikipedia....
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1answer
131 views

Horizon Problem: How did different regions of space acquire different temperatures?

I'm trying to understand the Horizon Problem. Inflation describes how non-causally connected parts of the universe (that we see today) could once have been in causal contact. The reasoning goes that ...
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How the universe came into existence? [duplicate]

I've nothing much to say here, except the fact that I just wanted to know what is the common or contemporary answers to this question.
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1answer
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Picturing the density inhomogeneity of matter at recombination

The CMB was formed at the time of recombination which suggests that the analysis of the temperature anisotropies of the CMB helps to infer the profile of density fluctuations at the time of ...
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1answer
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Is cosmic inflation slow rather than fast

According to cosmic inflation models, there is an early period where $\Lambda$ dominates and the scale factor grows exponentially. Among other things, this helps to give a reason for large-scale ...
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According to the actual understanding, was the Big Bang a singularity in space or rather in density? [duplicate]

Often in cosmology erroneous pictures are transmitted by the recurrent hearing from various sources or by sentences taken out from bigger discussion, and even by omitting adjectives and the likes. I ...