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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2answers
32 views

Origin of time or origin of space-time?

This might be a mere theoretical concept and I might even get down voted for asking this. I read in several sources that winding back general relativity equations leads to $t=0$, origin of time. ...
0
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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 ...
4
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1answer
332 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 ...
1
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0answers
43 views

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

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 ...
-1
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2answers
70 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
39 views

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

(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 ...
0
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4answers
47 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 ...
1
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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 ...
2
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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?
2
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0answers
55 views

Solving the Friedmann Equation [on hold]

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
41 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, ...
1
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2answers
61 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 ...
5
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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 $H_0$, $\Omega_m$, ...
1
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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 ...
0
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0answers
44 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 ...
3
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0answers
19 views

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/...
2
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1answer
63 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 ...
1
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0answers
45 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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votes
1answer
64 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
51 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 ...
0
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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, ...
1
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1answer
221 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 ...
3
votes
2answers
99 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, ...
1
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0answers
58 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
votes
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 ...
16
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3answers
248 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 ...
4
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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 ...
11
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3answers
497 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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2answers
62 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
44 views

Correct expression for Suv = Tuv - 1/2*guv*T in Chapter 8.1 of Einstein Gravity in a Nutshell [closed]

In the textbook "Einstein Gravity in a Nutshell", Chapter 8 is called "The Dynamic Universe" and employs a well known expression for the stress energy tensor Tuv for a perfect fluid filling the ...
4
votes
1answer
22 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 ...
15
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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 ...
0
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2answers
54 views

Does the expanding universe prove the theory of relativity wrong? [duplicate]

I recently read an article which said the the universe is expanding with a very high velocity, which is even a lot faster than the speed of light. So, doesn't this prove Einstein's theory wrong which ...
5
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1answer
52 views

What does the Friedmann Equation imply for the cosmological constant?

The Friedmann equation is given by $$(\frac{\dot{a}}{a})^2 = \frac{8 \pi G \rho}{3} - \frac{kc^2}{a^2} + \frac{\lambda c^2}{3}.$$ Considering that the left hand side is analagous to the kinetic energy ...
0
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1answer
69 views

Source of energy for infinite expansion

I read in several sources that matter, dark matter & dark energy contributes 5%, 26% and 69% of the known universe. Also, I read in a slide deck about Big Bang theory on nobelprize.org that total ...
1
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1answer
242 views

What is the meaning of the particle horizon in conformal diagrams?

I'm reading "Physical Foundations of Cosmology" (Mukhanov) and in Chapter 2.3 conformal diagrams get introduced. They seem to be a (graphical) tool to understand the causal structure of the universe. ...
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0answers
25 views

How do we know that the big bang happened everywhere, simultaneously? [duplicate]

If we can't see beyond the limits of the observable universe how do we know that the big bang happened everywhere, simultaneously?
6
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3answers
129 views

Two Robertson-Walker observers, at what time will a light signal be received?

Here is a question I have that is inspired by this question here. The spacetime metric of a radiation-filled, spatially flat ($k = 0$) Robertson-Walker universe is given by$$ds^2 = - dT^2 + T[dx^2 + ...
0
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1answer
67 views

If the universe stopped expanding, when would we realize it?

Suppose in this moment the universe stopped expanding, how long would if take and how would we register the change? Since the bodies are still and it is space that is expanding in between, we wouldn'...
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0answers
24 views

When was the first Earth-like planet formed?

By Earth-like planet I mean the formation of a planet near a Star analog to our Sun, almost the same size as Earth and all the parts needed to form cells. If we can pinpoint this Earth-like planet ...
0
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1answer
68 views

Which theoretical models are there between quantum mechanics and cosmology? [closed]

I'm an enthusiast/hobbyist right now and I'm quite curious about the subject of understanding which scales come between the quantum scale (ab initio/first principles) and the macroscopic scale. After ...
0
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0answers
15 views

Flat Universe and Evolving Geometry [duplicate]

I have heard in multiple locations that in the case of a flat universe, the universe is infinite. This seems to present multiple issues: a) If a flat universe were infinite, would would the space of ...
0
votes
1answer
105 views

How is the speed in Hubble's law determined, if not from the Doppler shift?

Hubble's law originally mentions the speed (i.e. proper motion) of a receding galaxy, as calculated via the Doppler effect from the observed redshift. However, the observed redshift is today explained ...
8
votes
3answers
270 views

Is the Boltzmann brain problem really clearly established as a problem?

Starting from the assumption of a cosmology in which Boltzmann brains dominate over evolved ones, it is not immediately obvious to me that there is a real problem, since only Boltzmann brains ...
3
votes
2answers
100 views

Is the universe actually expanding?

Up until recently I was fairly sure that the universe is expanding, i.e. the (spatial) metric is changing proportionally to the scale factor, such that the distance measured between objects is ...
0
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0answers
28 views

Resources on how to estimate the power density of gravity?

If a drop a ball from a height $h$, the work done by gravity (in this particular situation $i$) is $w_i=\int_0^h F_G \cdot dl \approx mgh$. What I would like, is to estimate the work done by gravity ...
6
votes
1answer
240 views

The cosmological constant as a Lagrange multiplier?

The cosmological constant $\Lambda$ can be introduced into the gravitational action like this : \begin{equation} S = \frac{1}{2 \kappa} \int_{\Omega} (R - 2 \Lambda) \sqrt{-g} \; d^4 x + \text{matter ...
6
votes
4answers
133 views

Is it reasonable to visualise an additional dimension of time as a part of 4th dimensional spacetime

[EDIT] In the spirit of asking a "good question", this question is considerably more refined that the one I decided to blurt out earlier thanks to a little additional research and nudges from comments....
5
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1answer
253 views

An argument that massive particles don't redshift?

I start with the spatially flat FRW metric in conformal co-ordinates: $$ds^2=a^2(\eta)(d\eta^2-dx^2-dy^2-dz^2)$$ This metric has the following non-zero Christoffel symbols: \begin{eqnarray} \Gamma^...
3
votes
2answers
38 views

A question on the redshift of photons due to cosmic expansion

Given that the universe is expanding over time, in the sense that the (spatial) metric is changing over time, corresponding to the physical distance between objects increasing, naïve intuition leads ...