# cosmology's questions - English 1answer

2.765 cosmology questions.

### 2 Does the Cosmic Microwave Background falsify relativity of velocity?

In special relativity velocity is relative and there is no absolute rest frame . However the cosmic microwave background radiation does have a rest frame. Earth is moving with 328 km/s with respect to ...

### What should the acceleration of the expansion of the Universe be for the CMBR temperature to stabilize?

Locally, it is generally easier to heat things than to cool them. However on large scales, the universe is cooling down. The cosmic microwave background radiation temperature corresponds roughly to ...

### How fine-tuned is the expansion rate of the universe?

I read somewhere that If the expansion rate of the universe were larger, no galaxies would form. If the expansion rate of the universe were smaller, the universe would rapidly collapse. Is it true ...

### 51 Red shifted to what?

I searched and found a lot of questions and answers about red shift here but none with the answer to mine. (sorry if it is there somewhere and I did not find it.) Everyone is saying the light from ...

### Gauge-invariance of curvature perturbation in uniform density gauge

The perturbed energy conservation equation is given by $$\delta\rho'+3\mathcal{H}(\delta\rho+\delta P)-3(\bar{\rho}+\bar{P})\psi'+(\bar{\rho}+\bar{P})\nabla^2(V+\sigma)=0.$$ If we substitute in the ...

### -2 Is the second law of thermodynamics evidence that the global universe had a beginning in time?

In modern form, the second law of thermodynamics states that the entropy of an isolated system either remains constant, decreases, or increases, but the major highlight being that it is overwhelmingly ...

### Space expansion and speed of light

I recently saw a video on gravitational waves that says that expansion of space can only be measured due to changes in speed of light as everything else that could have been used to measure the ...

### 1 Given redshift how do I obtain distance in parsec?

I have a catalog I am experimenting with that has some spectroscopic red shift data available (in the range from 0.5-1) as well as total Ks (K -short band) flux data available, how would I go about ...

### 2 How small could the universe be?

2 answers, 62 views cosmology universe curvature
In the article Will the Universe expand forever?, NASA states: We now know (as of 2013) that the universe is flat with only a 0.4% margin of error. This suggests that the Universe is infinite in ...

### 2 Can we have bubbles of the tenth dimension in our universe?

If the gap between the branes in the Big Bounce theory is the 10th dimension, and branes have wrinkles as they collide, can our universe have 'bubbles' in the 10th dimension? This is in reference to ...

### 1 How the cosmological scale factor $a(t)$ will have maximum value followed by a contracting phase of the universe?

The scale factor satisfies the following equation - $$\frac{\dot a^{2}}{a^{2}} + \frac{k}{a^{2}} = \frac{8\pi G}{3} \rho(t)$$ where $k$ is constant proportional to total energy of the dynamical ...

### 12 Integrated Boltzmann equation for dark matter

In Dodelson's book, the equation for a scattering process $a + b \Leftrightarrow c + d$ is given as \begin{align} a^{-3} \frac{\mathrm d (n_a a^3)}{\mathrm d t}&=-n^{\text{eq}}_a n^{\text{eq}}...

### 2 How do we know that the dark matter is cold or non-relativistic?

According to the $\Lambda$CDM parametrization of the Standard Model of Big Bang cosmology, the universe contains a cosmological constant $\Lambda$ associated with $73\%$ dark energy, $23\%$cold dark ...

### 10 A Cosmological horizon at the Hubble radius?

I have calculated that if one extends a rigid ruler into space by a fixed proper distance $D$ then a clock at the end of the ruler, running on proper time $\tau$, will run more slowly than time $t$ at ...

### -1 During the big bang if the universe was size of a baseball what did it expand into? [duplicate]

The edge of universe is beyond our reach and hence don’t know what it is expanding into. During Big Bang if it was the size of a baseball what did it expand into?

### Observers and traveling backwards in time in Godel's universe, Is it possible?

Once we choose a position in Godel's universe, there is a cylinder associated to our position which divide our spacetime into two separated regions and in this coordinate system, our cylinder have a ...

### The theoretical calculation underlying the cosmological constant problem

The electroweak vacuum energy density is $10^{120}$ times larger than the currently observed value of the dark energy. I have two questions in regard to this statement. $\bullet$ Firstly, what are ...

### Can Dark Matter be light and cold at the same time?

1 answers, 52 views cosmology mass dark-matter
In most of the papers that I encounter, the mass range for Dark matter is generally taken to be 1-1000 GeV. Is it possible that dark matter is of the order of few MeV? What observation is violated in ...

### 2 Which extensions of the Standard model have the inflaton candidates?

Like dark matter and dark energy, there is no candidate of inflaton in the Standard Model (SM) of the particle physics but its various extension does have candidates of dark matter. Does the minimal ...

### 1 Can we see beyond the Hubble Volume?

The wiki article on the Hubble volume says that the Hubble volume is often confused with the limit of the observable universe. This thread: When will the Hubble volume coincide with the volume of the ...

### 6 Why doesn't the light from galaxies appear stretched? [duplicate]

Maybe it's my ignorance of astrophysics/cosmology, but I have been wondering this: Why do galaxies not appear stretched when we observe them? Assuming a galaxy that we observe is 100,000 light years ...

### Power spectrum and correlation function?

Imagine a Universe such that the dark matter correlation function is a Gaussian centered around $0$ and with variance $\sigma^2$. How should one proceed to obtain the power spectrum of such an ...

### 1 What effect would result from always keeping Page time >> Age of universe?

A thought experiment: I imagine, similar to early theories (eg: Hoyle), that the universe’s mass is not constant, but increases over cosmic time, in such a way that even the thinnest vacuum will ...

### 5 Connection between BAO und CMB Spectrum

I have a problem understanding the connection between the accoustic peaks in the CMB spectrum and the baryon oscillation picture. On the one hand it is stated, that the odd accoustic peaks (1,3,5..) ...

### 1 What is Cosmic Downsizing?

I've had a quick look at a few lecture pdfs and papers as supplementals to my own given lecture notes, but I can't seem to get a proper explanation for what cosmic downsizing is. The closest ...

### 2 Do quantum fluctuations in the inflaton field lead to fluctuations in the potential energy density?

During inflation there are quantum fluctuations in the inflaton field. Do quantum fluctuations in the inflaton field lead to fluctuations in the energy density?

### 2 Hadron contribution to effective degrees of freedom in early Universe

In transition from quark-gluon plasma to hadron gas in the early Universe, the value of the effective degrees of freedom $g_{\star}$ decreases abruptly. This seems to me like a sort of decoupling: ...

### Space expansion and conservation of energy [duplicate]

The phrase of 'expansion of universe' , as we are led to believe, is mostly referring to an expansion in space. But is space expanding in the sense that the space between two particles is increasing ...

### 2 Why does the Hubble constant have a subscript zero?

The title says it all. Why is the Hubble constant written $H_0$ and not just $H$? What is the purpose of that subscript $_0$?

### 2 Intuitive understanding of the virial radius/mass

1 answers, 36 views cosmology galaxies virial-theorem
I'm currently trying with no avail to understand the intricacies that define the virial radius or mass in a way that is different from the actual galactic radius/mass. I understand it is derived in ...

### 2 Second law of thermodynamics implies a linear cosmology?

If one applies the second law of thermodynamics to the Universe[1] as a whole then one might expect that the entropy of the Universe always increases as time goes forward (or more accurately that the ...

### Can the age of the universe at the time of emitted light be calculated with Hubble's Law?

Let there be an object being observed, distance D away. D equals the speed of light times the time it takes the light to arrive (but space expands while the light is propagating, which is most likely ...

### 4 Attractive gravity has negative energy, what about repulsive gravity in the inflation phase?

Alan Guth's cosmic inflation theory posits that about 10^-38 second before the Big Bang which led to our particular universe, a tiny patch of space doubled in size more than 100 times, from sub-atomic ...

### -1 Is it possible to experimentally show that observable universe is much larger than ~93B ly?

Thought experiment: Entangle a pair of particles. Package one half of each pair in a special package, and send it out in a nano-spaceship keep the other half of the pair of entangled particles here ...

### 2 Pressure in Scott Dodelson's book Modern Cosmology

I am completely at a loss in deriving the equation 2.62 given in Scott Dodelson's Modern Cosmology book. That is the following expression for the pressure of Fermi-Dirac and Bose-Einstein ...

### 16 How did we 'discover' dark matter? [closed]

I'm an astrophysics student and I've been researching this topic and there is one point that keeps eluding me. How did the scientific community realize that there had to be dark matter in the ...

### -2 Could this experiment show how big the universe really is? [duplicate]

Thought experiment: Entangle a pair of particles. Package one half of each pair in a special package, and send it out in a nano-spaceship keep the other half of the pair of entangled particles here ...

### 12 Does the fact that energy is not conserved in cosmology open the possibility of new matter/atoms being created in the universe?

Does it open the possibility of new baryonic matter/atoms being created in the universe and avoiding the Heat Death of the universe?

### 1 How did cosmologists calculate the small density of dark energy in our universe?

I've seen the number a few times (138 x10 to the power of -123) but i haven´t found how scientists calculated it. Max Tegmark mentions it in his research paper about multiverse types as representing ...

### 1 Size of the Universe at end of recombination (~477000 years)?

2 answers, 38 views cosmology big-bang
What was the size of the Universe at start of recombination (377000 years) when the Universe started to become transparent and release of trapped photons into what we now know as the CMB and till end ...

### Would QCD phase transition happen earlier if QCD was asymptotically safe?

It is well established that QCD is asymptotically free i.e at high energy scales the QCD coupling tends to zero. Hypothetically, what if QCD was not asymptotically free and instead asymptotically safe?...

### -1 Which was first, energy or matter in the creation of our universe?

1 answers, 2.314 views particle-physics cosmology big-bang
Was it the Big Bang or was it something else that gives us our universe in its present condition? Did it all begin with just pure energy that eventually evolved into simple atoms of matter, that ...

### Is there a difference between horizon and comoving distances?

0 answers, 20 views cosmology metric-tensor distance
I wonder if there a difference between horizon and comoving distances in cosmology ? I mean both correspond to a null geodesic $(ds^2=0)$ right ?