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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Brans-Dicke vacuum cosmology?

Has anyone heard of papers describing Brans-Dicke vacuum cosmology solutions? I'd be interested to know if the following calculations make sense. The Brans-Dicke action for a Universe dominated by ...
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On average, how many atoms are in there in a molecule?

I was doing some Fermi style questions and one of them got me thinking about how many atoms are in the average molecule. Since hydrogen is the most abundant, and hydrogen is diatomic I guessed around ...
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What relation exists between Wolfram's model of the Cellular Automata universe and the Holographic Principle and the Bekenstein Bound?

Are computational models of the universe, like Stephen Wolfram's cellular automaton model of the universe (proposed in his book A New Kind of Science), somehow related to the Holographic Principle and ...
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Do cosmic voids have to exist to prevent gravitational collapse of the Universe?

Since a black hole radius is 2 times the mass, it seems like any infinite distribution of matter has to have voids of increasing size as the scale changes in order for the whole Universe not to ...
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How much has the universe expanded?

Expansion speed of universe is 68 km/sec, which is 1/4400 times the speed of light. From the big bang 13.7 billion years ago, the universe would have expanded a few hundred million light years at ...
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Light branes and swampland/landscape

Following a talk by Cumrum Vafa, I understood that light branes are important as he quoted the formula: $$m\sim \exp(-\alpha M/M_P)$$ and then, he stated that light massive particles AND extended ...
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76 views

Geometry of Big Bang

As you look at circle cross-sections around a sphere it's obvious that as you move the circles along the z-axis, the circles get bigger and smaller. When you're at the farthest points on the $z$-axis, ...
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152 views

Shape of the observable universe or Cosmic Horizon

My question is about the shape of the observable universe or Cosmic Horizon. In literature it is described as having a radius, constant. But in an accelerating expanding universe this seems ...
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63 views

Is the gravitational pull of the early ($10^{10}$ more massive) universe still observable?

During one of the early epochs of the universe it used to be 10^10 times more massive than it is now, just before the matter / antimatter annihilation. So what cosmic gravity is being asserted on the ...
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60 views

Rates of reaction and the expansion of the universe

In Moore, 2013 the following statement is given about reaction rates and the expansion of the universe (my wording): For a reaction to occur we need a rate corresponding to at least one reaction ...
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177 views

Finding equation for energy density of matter

I'm trying to get an equation for the energy density of matter of the universe $\rho(t)$, assuming the number of particles is conserved and rest mass energy is much greater than kinetic energy. $\rho(...
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157 views

Should Einstein's Field Equations be modified when using conformal time?

Let us consider the FRW metric for flat space expressed in terms of conformal time $\eta$ and cartesian spatial co-ordinates $x,y,z$: $$ds^2=a^2(\eta)\{d\eta^2-dx^2-dy^2-dz^2\}.$$ As in the standard ...
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437 views

Why we do not see beyond the cosmic horizon?

I've read that is because light hasn"t had time to reach us? Would it someday reach us and would we be able to see beyond the horizon?
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219 views

Nomenclature “Material System”

This is just a quick question about nomenclature from General Relativity. Most textbooks refer to "material systems"; 1) Does a material system have to be a system with mass? or can it just be a ...
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66 views

In the string theory multiverse, is every universe fated to collapse in a Big Crunch?

This is what I understand of the multiverse of the string landscape: the "laws of physics" reside in a 100-dimensional string landscape, which has ~$10^{500}$ minima. Each of the minima corresponds to ...
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724 views

Most stationary object in the universe

There is an object in the universe for which the Sun appears to be more stationary as it travels through the universe than a star on some other system. There is probably another object for which both ...
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164 views

Why Does the amount of gravitational potential energy in the universe increase as it expands?

Ok so this is my thought process... please bear with me I understand that as the universe expands, there is supposed formation of matter due to dark energy,and with an increase in mass comes and ...
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55 views

Repulsive gravity formulation: a query

I was going through this arxiv paper by Magnano: gr-qc/9312008 For modeling non linear and repulsive gravity he takes the generic lagrangian $$L = f(R)\sqrt{-g}.$$ Varying this $L$ with $g^{\mu\nu}$, ...
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97 views

Red shift and the expanding universe pertaining to gravity wells

is it possible that gravity's effect on the energy level of light could be causing the red shift in light arriving from other galaxies and not an expanding universe? it is impossible, after all, to ...
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128 views

Many worlds interpretation of quantamechanics vs eternal information

Whats the main difference between the theory of many worlds interpretation in quantum mechanics and the theory of eternal inflation or are they the same in regard of their vision to the multiverse ?
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167 views

Is entropy of the universe constant?

Studying the thermodynamics of the early universe we arrived at the conclusion that entropy of the universe has to be constant for the entropy to be a function of state. But from basilar ...
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175 views

Tired light question

How do we arrive at the conclusion that redshifted light from cosmological sources has 'lost' energy during its transit from source to us? If in fact no actual measurement of the energy of the ...
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128 views

question about other dimensions

The Big Bounce theory says that before the Big Bang there were other Big Bangs over and over. For example, the universe would have a Big Bang within it and then this new mini-universe would expand, ...

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