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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Gravitational time dilation caused by a galaxy, and by

In a word, if you are sitting on the Earth, if I'm not mistaken you are experiencing Time Dilation compared to being in deep solar system space. Due to the mass of the Earth. However. We're all ...
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0answers
43 views

Covariant Fluid Flow approach

I am doing Cosmological perturbation. Currently reading a paper by Bruni et al. In that it is mentioned that they are using covariant fluid flow approach to cosmology. Can any body give me a rough ...
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0answers
71 views

Topological implications of symbolic represenation of the relativity

I have seen in the online Stanford Encyclopedia of Philosophy in the entry on Copenhagen Interpretation of Quantum Mechanics that Niels Bohr had argued that the theory of relativity is not a literal ...
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1answer
83 views

What fraction of baryonic matter is in stars?

We know from big bang nucleosynthesis that baryonic matter accounts for about 5% of the universe's total mass-energy density. What is the current best estimate of how much of this is in the form of ...
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0answers
30 views

Where did the energy of the universe come from? [duplicate]

Last week my Physics teacher asked the class this question: If the internal energy of the universe is constant, then where did all the energy come from originally?
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2answers
127 views

Could these hypothetical very long distance properties of light offer alternatives to an expanding universe or a finite number of galaxies?

Assume that there could be a property of light that it gets very gradually steadily red-shifted by traveling large astronomical distances, in exactly the same way that Doppler shift from receding ...
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0answers
37 views

Is There A 'Leading Edge' to the Universe? [duplicate]

Is there a center to the universe? Did the Big Bang happen at a single location, spreading outwards? Or, was the Big Bang more of a state-change, in which the universe went from super-dense and ...
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2answers
119 views

If the universe is spatially infinite, and if something (A) is possible, does that mean the thing (A) happens?

Assuming that the universe is spatially infinite, extending outwards in all directions without end, and is consistent with what we have thus far observed, and predict to find (and predict to be out ...
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3answers
883 views

Other explanation for cosmological redshift?

I'm interested if any of the following explanations have enough predictive capability to explain the observations we see today. The claim is that the Universe is not expanding, and that red-shift of ...
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1answer
155 views

Redshift and conformal time

What is the relationship between redshift and conformal time ? For example in a paper i found: taking $z_e = 3234 $ at the time of radiaton-matter equality yields the conformal time ...
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1answer
127 views

Do Quantum Fluctuations happen evenly through universe?

I was wondering if Quantum Fluctuations are completely unpredictable, but do our observations tell us if these fluctuations happen evenly through space or are there regions where more quantum ...
3
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1answer
164 views

Is the Universe still believed to be flat?

I have read a handful of old articles from mid 2013 expressing that the Universe may, in fact, be curved. http://www.nature.com/news/universe-may-be-curved-not-flat-1.13776 ...
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1answer
74 views

How the distance and diameter of the stars are experimentally measured?

How to measure the distance of a star and the diameter of it from earth based tools? Normally we can measure the angle between the stars edge. But diameter or distance needed to measure the other ...
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3answers
365 views

Recommended books for a “relativity for poets” class?

I teach physics at a community college and have developed a new course titled "Relativity for Poets," which I will be teaching for the first time in spring 2015. As implied by the title, it's a ...
2
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2answers
122 views

Where's all the light going in the universe?

According to my knowledge, matter can not go faster than light, and light is emitted constantly in the universe from many types of reactions. If this is so, then isn't light constantly getting past ...
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1answer
106 views

Concerning the transtition between matter dominated universe and radiation dominated universe

What caused the transition, and how to we know that this did not warp our calculations for the age of the early universe ?
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3answers
1k views

Failure of the Steady State Theory

I was reading a journal of astronomy and came to the most famous opponent of Big Bang theory: The Steady State Theory: The 20th - century theory was proposed by Hoyle,Gold and Bondie. The ...
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1answer
68 views

Increasing matter density in the Friedman equations?

From what I understand, the Friedman equation is $H^2=\frac{8\pi G}{3} \rho$ and the $H$, the Hubble constant, is a measure of the rate of expansion of the universe. What I am confused about is that ...
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1answer
164 views

Infinite space, nothingness, and the concept of infinity [closed]

These two questions are a follow-up to my previous Phys.SE question on finite space. The first question challenges the concept of infinite space. Space is something: it is ruled by laws, and ...
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1answer
132 views

Infinite space or finite space?

I have been reading about the nature of space (Brian Greene's and Max Tegmark's recent books) and have a question on the size of space. Both references acknowledge that general relativity equations ...
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1answer
149 views

What does the “size of the universe” mean if the Universe is infinite?

There are questions that may seem to be similar to this one, but I've yet to find an answer. I have come to understand that a flat universe, that is to say a curverature of $k=0$ which means that ...
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1answer
61 views

Determining zero energy from $k=0$?

I'm curious as to the equations necessary for finding a total energy of 0 (or, I suppose, the energy density of empty space due to quantum fluctuations) in a flat Friedmann universe such as ours. The ...
2
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1answer
127 views

The 3rd derivative of the size of the universe?

We know the size of observable universe, that it is expanding, that expansion is accelerating. Does sufficiently precise data exist to determine at least the SIGN of the next (the 3rd) derivative? ...
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1answer
60 views

Size of the Hubble volume in 4 billion years

What will the size of the observable universe be in 4 billion years? At present day, light would only reach us if it came from a galaxy within 13.5 billion1 light years, with anything outside that ...
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0answers
96 views

Center of mass & Big Bang Theory

I have read in my text books about the Big Bang theory and quote "the entire matter in the universe was once concentrated in a single very dense and hot fire ball; it exploded some 20 billion years ...
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1answer
64 views

Quasars with differing emission / absorption redshifts

I was reading this (perhaps offbeat) "anti-big-bang" article, http://www.newtonphysics.on.ca/hubble/ and the author comments "a recent catalog of objects having very large redshifts shows that among ...
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2answers
83 views

Newtonian potential defines size of Universe?

As I understand it the Newtonian potential $\Phi$ at the position of an observer in space is approximately given by: $$\Phi \sim \frac{G M}{R},$$ where $R$ is the radius of a sphere centered at the ...
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3answers
102 views

source of energy for expansion of the universe

I know in order for something to expand or grow it has to be fueled by something? So, if the universe is expanding their must be something that's keeping the universe at a temperature high enough for ...
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1answer
94 views

dark energy and expansion definition [closed]

I am trying to get a few key points correct regarding expansion and dark energy... Expansion: the Universe has been expanding continuously at different rates since the singularity that caused the ...
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0answers
31 views

Need advice for self study in physics [duplicate]

I need advice for self study in Physics. I have a good theoretical and practical background in math & statistics, but virtually no physics. My ultimate interest is cosmology / relativity / ...
4
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2answers
193 views

FRW metric and its validity througout the age of the universe

Why do we think that the FRW metric should be valid throughout the entire history of the universe?
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1answer
55 views

TEO: raise of a System [closed]

I think this question is settled somewhere betweeen physics and philosophy and we surely can not find a answer that fits 100%, but lets talk about it. While thinking about how the universe and its ...
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1answer
174 views

How can the Big Bang singularity exist in spacetime if it has zero volume? [closed]

How can the Big Bang singularity exist if it has zero volume? I tried googling to find the answer - no help. Can someone give a general idea how can the big bang singularity exist even if it has zero ...
4
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2answers
176 views

Could Dust Equation of State have some negative pressure?

Traditionally the cosmological equation of state of cold matter (so-called dust) is simply: $$p = 0.$$ But, in Newtonian terms, each particle is gravitationally attracting every other particle. ...
2
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1answer
75 views

Why is the graph of CMB/black-body radiation asymptotic?

Speaking of this graph of blackbody radiation, I see that the graph goes to 0 asymptotically: As we go to higher and higher frequencies, the energy of a single photon becomes increasingly high. ...
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0answers
119 views

How strong is the force of space expansion?

There are many questions about space expansion, its cause, or its effects. But I have the feeling we never get straight and simple answers. I do not expect answers to be simple in general, but I ...
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3answers
1k views

Why is the sky dark at night? [duplicate]

The question is the well-known Olbers' paradox: If there are so many/infinite stars, in every direction, why is it dark at night? Where goes all the light we don't see? Discussing with a friend ...
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1answer
44 views

Are there other places to look for the gravitational wave, other than space?

The recent BICEP2 announcements about the existence of gravity waves made the news, and made a big pitch for cosmic inflation, in the process they also claimed to detect the existence of gravitational ...
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1answer
143 views

Spiral galaxies and gravity lenses

Spiral Galaxies must have a great deal more mass than elliptical galaxies of the same size in order to account for the flat velocity curve. I've seen references of eight to ten times the visible ...
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4answers
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Did the Big Bang happen at a point?

TV documentaries invariably show the Big Bang as an exploding ball of fire expanding outwards. Did the Big Bang really explode outwards from a point like this? If not, what did happen?
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1answer
128 views

A universe from nothing [duplicate]

The more time you spending in measuring your experiment (thus standard deviation will become smaller) the more precisely you will measure energy of this system.... energy time uncertainly principle ...
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3answers
209 views

Has the universe we live in started as a black hole that is imploding?

As discussed in this question, as far back as the 1960s it was suggested that the Schwarzschild metric can be smoothly joined to a de Sitter metric. For example the idea has been used by Lee Smolin in ...
2
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1answer
81 views

Can a lone black hole in a closed Universe evaporate?

If there is a closed Universe which only has a black hole in it, can that black hole evaporate? As the black hole evaporates, it gives off energy, which will eventually come back and be re-absorbed ...
4
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1answer
723 views

Why do we only talk about three options when it comes to the Shape of Spacetime?

The shape of spacetime is a hotly debated topic which can determine how we understand the ultimate fate of the universe and many more things. Despite this I only ever see factions supporting three ...
9
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1answer
636 views

How does the Hubble parameter change with the age of the universe?

How does the Hubble parameter change with the age of the universe? This question was posted recently, and I had almost finished writing an answer when the question was deleted. Since it's a shame to ...
3
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0answers
60 views

When Did Structures Begin to Orbit in the Universe?

In Big Bang Cosmology, I am familiar with the radiation era, and the moment of last scattering, but I am just not familiar with the physics of orbits, or of the “falling outward,“ if you will, that ...
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1answer
66 views

Simplifying Friedmann's Equation

So we have one of Friedmann's equation: $$\rho_c = \frac{3H^2}{8\pi G}$$ Using This website, resources where gathered for specific times in the universe. The resources being the Hubble constant at ...
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0answers
47 views

A physical understanding of the 2/3 in the Sachs-Wolfe effect

I am trying to develop an intuitive but quantitative understanding of the factor of 2/3 in the Sachs-Wolfe effect. I believe I have a picture that makes sense but it relies on one assumption I don't ...
0
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1answer
61 views

Friedmann equations question

Friedmann equations for critical density is: $$\rho_c = \frac{3H^2}{8\pi G}$$ Is there any other way to write this equation? For example: $$\rho_c = \frac{3}{8\pi GH^2}$$ I saw the above form on ...
3
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1answer
181 views

Is there an alternative to dark matter?

I've been reading about dark matter. I understood that its existence is inferred by the discrepancy between the gravity exerted by a galaxy (total mass) and the movement of the stars that compose it. ...