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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Is a light wave's amplitude stretched, along with the “red shift” stretch - making it brighter?

When light waves are stretched and "red shifted", is the amplitude of the light wave stretched as well, affecting the intensity/brightness of the light wave?
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5answers
3k views

Scale factor and age of the universe

Why is the cosmological scale factor (expansion rate of the universe) not simply the time $t$, i.e. the age of the universe?
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0answers
23 views

Collision term in Boltzmann Equation

In Dodelson's Book, chapter 3, we have the collision term in Boltzmann equation is written as $$\int\frac{d^3p_1}{(2\pi)^32E_1}\int\frac{d^3p_2}{(2\pi)^32E_2}\int\frac{d^3p_3}{(2\pi)^32E_3}\int\frac{...
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1answer
36 views

Why is energy minimized over the string landscape?

As understand it, the 4D string landscape is a function that assigns an energy to every possible compactification of the 6 small spatial dimensions. We expect our universe to lie in a local energy ...
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1answer
34 views

Have conservation laws stayed constant since the Big Bang?

By conservation laws I mean quantum number conservation, such as baryon and lepton numbers. As far as I know, we have a reasonable confidence that experimental data confirms a lot of features of our ...
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0answers
31 views

Why would be axions be a candidate for warm/cold dark matter?

Axions are the current main favoured candidate for the identity of dark matter. While their masses are sub-$\mathrm{eV}$, why is it that they would be cold or possibly warm dark matter candidates ...
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1answer
29 views

Cold dark matter halos and cusps

Cold dark matter is one of the preferred candidates for dark matter. Yet, models such as the NFW and isothermal predict that halos will contain cusps, where the density tends towards infinity, despite ...
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0answers
17 views

How would shape noise be distinguished from weak gravitational lensing?

Weak gravitational lensing distorts the image of galaxies by making them elliptical and non elliptical, but this we see in every day effects. So how do you determine what is just the shape noise of ...
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3answers
111 views

Is this explanation for the Universe's low entropy at its origins plausible? [on hold]

I've been thinking about this, and it may sound very ludicrous (and it probably is), but I may have come up with a proto-explanation for it. I hope I won't come across as some misinformed perpetual ...
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1answer
157 views

Universe spawned from the death of a hyper black hole?

Some Cosmologists have speculated that the Universe formed from the debris ejected when a four-dimensional star collapsed into a black hole a scenario that would help to explain why the cosmos seems ...
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2answers
87 views

Origin of time or origin of space-time? [on hold]

Are there alternate theories or models that contradicts the idea that big bang is the origin of time ?
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1answer
58 views

String Theory Vs. Holographic Theory of the Universe. Compatibility [on hold]

String Theory claims strings to be one dimensional objects vs. dimensionless point particles of the Standard Model. String Theory also needs 10 or possibly more spatial dimensions to operate in. ...
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6answers
7k views

Why was the universe in an extraordinarily low-entropy state right after the big bang?

Let me start by saying that I have no scientific background whatsoever. I am very interested in science though and I'm currently enjoying Brian Greene's The Fabric of the Cosmos. I'm at chapter 7 and ...
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2answers
103 views

What kind of effect would cosmic expansion have on planetary motion or keplerian orbit? [duplicate]

The question is, if two following phenomena are thought to effect to the motion of the planet that orbits a star: Celestial body is pulled by some massive star by ordinary Newtonian gravity: $$ m \...
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2answers
83 views

Source of Dark Energy.

In accelerated expansion dark energy play a crucial role because of negative pressure, but during expansion Dark Energy(DE) do not dilute. DE density is independent of scale factor unlike matter and ...
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0answers
34 views

Mechanism for collapse of iron stars into black holes via quantum tunnelling

In the wikipedia page "Future of an expanding universe" it refers to the scenario of a future without proton decay. The page talks about how processes would lead to stellar-mass cold spheres of iron, ...
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3answers
59 views

What was the pressure of the Universe, when the CMB was 300K?

I suspect, at the time when stars didn't exist yet, we could calculate with largely homogeneous Hydrogen-Helium mix. Unfortunately, the sources I've found on google aren't really detailed.
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0answers
15 views

Did the universe really start with a single point? [duplicate]

I read everywhere the famous, but offhand statement that "the universe began from a single point" and this bugs me because that surely isn't true. My understanding has always been that this was an ...
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2answers
101 views

Interpretation of cosmological redshift

I was trying to understand why we cannot explain the observed redshift of distant galaxies using special relativity and I came upon this article by Davis and Lineweaver. Unfortunately when I arrive ...
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1answer
153 views

Distance between two galaxies of different redshift

Let $Q_1$ and $Q_2$ two different objects in the Universe (we can think to two galaxies or quasars), that we observe from the Earth at different angular position $(\alpha_1,\delta_1)$, $(\alpha_2,\...
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3answers
108 views

How the speed of light is constant with the particle horizon moving toward us?

I asked a question earlier with the same subject in mind. However, I was rather upset at the time and it didn't make much sense. I'm taking an astronomy course and seem to be misunderstanding ...
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1answer
336 views

Has the cosmic alignment of Earth and the Solar System been explained (CMB anisotropy)

The Cosmic Microwave Background anisotropy is currently aligning Earth and the Solar System with the largest and earliest structure of the universe. Has this been explained yet? Or are the theories ...
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0answers
44 views

How Higgs Field is a constant in an Expanding Universe [closed]

Going through information on Higgs Field in various forums, it seems that the mass of an object is due its interaction with the Higgs Field which is constant throughout the universe. However it ...
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2answers
76 views

Universe as a black hole - how did everything escape? [duplicate]

While listening to physics lectures, they glossed over things escaping from black holes insinuating it doesn't happen other than radiation... When talking about the period between Planck Epoch & ...
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0answers
41 views

Are we sure the universe expands uniformly in every direction? [closed]

(Now I know this has been asked before and this question will be removed probably as a duplicate by John Rennie.) Doesn't information from the CMB radiation give us a clue as to whether the ...
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4answers
52 views

Does the universe expand in every direction evenly? [duplicate]

I've heard that the universe is expanding constantly and that galaxies are moving further and further away from each other because of this. However, does the universe expand in every direction evenly ...
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2answers
114 views

Do Galactic Black Holes and Extra-Solar Planets change the mass balance of the universe?

With all the recent discoveries of supermassive black holes being at the center of nearly every galaxy, and the proposed "at least one planet" around (probably) every star. Does this affect the ...
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3answers
86 views

Why didn't helium continue to generate after a few minutes post-Big Bang?

Helium stopped generating after a few minutes after the Big Bang, but what made it impossible for its generation to continue?
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0answers
55 views

Solving the Friedmann Equation [closed]

Through substituting for values of $\rho$ and $k$, I have: $$H^2=\left(\frac{\dot{a}}{a}\right)^2=\frac{8\pi G C}{3a^4} + \frac{\Lambda c^2}{3}$$ $a=a(t)$, and $a(t=0)=0$. Note that $C$ is a ...
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1answer
44 views

Why is it said that photon-wavelengths have increased by a factor of 1000 since our universe became transparent to light?

After reading several explanations for the so-called "Hubble-radius", and still being confused, (as I reckon are some of the folks who tried to answer THAT question !!), I have a related question, ...
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2answers
62 views

Why is the possibility of the universe having a centre around which everything orbits not considered feasible?

I'm sure there is some kind of reason as to why this isn't considered as a legitimate theory but upon giving it though it seemed to make sense to me. I figure if all gravitational orbits are ...
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1answer
3k views

Where is radiation density in the Planck 2013 results?

I've been looking at the Planck 2013 cosmological parameters paper, trying to update my toy cosmology simulator with the most recent data. Most of the interesting values such as $H_0$, $\Omega_m$, ...
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1answer
31 views

Estimation of Location of First Acoustic Peak

This question is inspired by the page at NED and references it throughout the question. On the linked page there is a method for calculating the location of the first acoustic peak of the CMB. It ...
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0answers
45 views

Is there any primordial gravitational wave(form) that could explain dark energy?

First, I asked myself the question: Is there any gravitational waveform, however fantastical, that could explain dark energy. I suppose for a ubiquitous gravitational wave to be an explanation for ...
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0answers
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Where does this data comparing the CMB with Eddington's 'temperature of space' come from?

Eddington estimated the temperature of space assuming that star light would be scattered by interstellar dust. He came pretty close to the temperature of the CMB. http://www.astro.ucla.edu/~wright/...
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1answer
66 views

How can the size of the universe change over time when time is part of the universe?

Bit of an awkward way to phrase it, but basically: Time in relativity is just one of four dimensions of space-time. Nothing really special about it. Yet the universe was once smaller than it is now ...
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0answers
47 views

The expansion of the universe and its edge

I have understood that the universe is expanding at an accelerated rate. I have also thought, perhaps mistakenly, that this expansion means not that the masses in the universe are moving away from ...
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1answer
65 views

Is the universe actually formed according to the big bang theory? [closed]

Big-Bang pretty much explain the origin of universe, but still many people don't believe it. Isn't it possible that our universe come out of a white hole which have been pulled out from a black hole ...
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0answers
53 views

Are the laws of physics and the properties of matter intrinisc to the universe? Could it have been different? [closed]

Are the specific laws and properties of all that exists within this universe intrinsic to what makes up the universe (whether it was in the state it was in before the Big Bang, how it is now, or how ...
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0answers
13 views

Deceleration parameter in a Universe dominated by the cosmological constant

See Deriving the deceleration parameter of a universe whose density is dominated by matter for a derivation of $q_0 = \frac{\Omega_m}{2}-\Omega_\Lambda$ If the universe were $\Lambda$ dominated, ...
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1answer
223 views

Is the Universe infinite or it's just expanding to infinity?

According to the WMAP data in the past years we can say that our universe is considered flat or at least nearly flat and we also know that flat universe is allowed to be both infinite or finite in ...
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2answers
102 views

How does critical density affect the expansion of the universe if gravity is the curvature of space-time?

From what I know there are three scenarios about the end and expansion of the universe that all depend on the concept of critical density: If the matter of our universe is above critical density, ...
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0answers
59 views

Are all particles in the universe quantum-entangled on some level?

The CMB indicates the universe was in a thermal equilibrium early on, which I understand indicates causal connection. There appears to have been some rapid expansion, and the universe now appears to ...
3
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1answer
84 views

What is the name of matter with $w = - \frac{1}{3}$?

What kind of "matter" is described by the following thermodynamic relation ? \begin{equation}\tag{1} p = -\, \frac{1}{3} \; \rho, \end{equation} Where $p$ and $\rho$ are the pressure and density ...
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3answers
250 views

What does the cosmic neutrino background look like today, given that neutrinos possess mass?

This question is inspired by (or a follow-up to) the threads Where are all the slow neutrinos? and Is it possible that all “spontaneous nuclear decay” is actually “slow neutrino” induced? The cosmic ...
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2answers
42 views

Reference frame for the Cosmic Neutrino Background

It is well known that there exists a reference frame where the total momentum of the Cosmic Microwave Background is zero (a basic fact of special relativity applied to a collection of massless ...
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3answers
500 views

How would cold neutrinos get trapped by stars?

Continuing on from the cool physics Q&A'd on the threads Where are all the slow neutrinos?, Is it possible that all "spontaneous nuclear decay" is actually "slow neutrino" ...
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1answer
23 views

Deriving the deceleration parameter of a universe whose density is dominated by matter

Assuming that the density of the universe is dominated by matter, so that $\rho = \rho_m$, how can the deceleration parameter today be shown to be $q_0 = \frac{\Omega_m}{2} - \Omega_{\Lambda}$ The ...
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2answers
4k views

According to Hubble's Law, how can the expansion of the Universe be accelerating?

Scientists today think the expansion of the universe is accelerating. According to Hubble's law, objects further away are moving faster than objects closer to us. The further away an object is, the ...