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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698 views

The inner workings of the Olbers paradox

A long time ago I was told that the universe is finite. The provided "proof" (or reasoning), known as Olbers' paradox, was that on infinite universe there would be an infinite number of stars, and ...
9
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1answer
259 views

How deep is the region near an event horizon where Hawking radiation is generated?

In other words, how strong does gravity have to be to cause Hawking radiation to occur?
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0answers
353 views

Dimensional analysis to estimate order of magnitude of quantities

In the Coursera course From the Big Bang to Dark Energy on several occasions dimensional analysis was used to estimate the scale of quantities. This almost seems like a contradiction in terms to me, ...
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1answer
1k views

Is “A Brief History of Time” still up to date?

I have recently found my copy of Hawkings' A Brief History of Time which I have never finished. This time I'm determined to read it all the way through. However, the book is now almost 25 years old. ...
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2answers
89 views

Red Shift and Light Speed

I read several questions/answers about red shift on this web site, but there is something I still don't understand. Why is red-shift considered to be the effect of expanding space-time, why it could ...
2
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2answers
227 views

Is inflation deterministic?

In some theories inflation is supposed to be able to turn quantum fluctuations into macroscopic inhomogeneities. I don't understand how an isolated system such as the universe can undergo such a ...
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1answer
161 views

How does pressure relate to cosmological expansion?

In my cosmology class, we've been talking about pressure in the Friedmann Equation for acceleration: $$ \frac{\ddot{a}}{a}=-\frac{4 \pi G}{3} \left(\rho+\frac{3p}{c^2} \right) $$ In particular, I've ...
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0answers
91 views

Calculating the age of the Universe now and 30 billion years from now [duplicate]

I am confused about the age of the Universe. If you calculate it by $1 / H_0$, won't the answer be roughly the same today as it will be 30 billion years from now? I know $H_0$ is a parameter, not a ...
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1answer
130 views

Cosmological redshift in a lab?

I want to consider a thought experiment. Lets ignore technical problems of actually performing such an experiment. Consider two photons having the same wavelength. We send 1 photon to a distant ...
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1answer
96 views

Is gravity reversed during inflation?

I am not clear on that. Is gravity repulsive during inflation?
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2answers
158 views

Does the black hole only increase?

I don't understand this: The more mass a Black Hole has, the more gravity it create! The more gravity it has, the more mass it can get! With this lines, someday, all the universe will be a black ...
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1answer
284 views

Does the big bang violate the conservation of energy? [duplicate]

It is a fact that a thing is existing now because it had already been created. So why don't we take this to account to redefine law of conservation of energy.
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3answers
179 views

What happened to high temperatures in the universe?

We know that after the big bang the temperature was about 10^32 K. But now the average temperature of the universe is about 4 K. What happened to the temperature at that time? Where did it go?
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2answers
208 views

What is the universe expanding into? [duplicate]

Due to curiosity, it made me wonder what is outside of the universe, is it a new chemical or just empty space? But empty space have protons appearing and disappearing in it, it basically still has ...
3
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2answers
139 views

Do bubble universes have a centre?

This question is motivated by Big Bang Physics/Cosmology. If eternal inflation is a good description of the universe and we live in a bubble universe then presumably our bubble nucleated at a ...
1
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1answer
726 views

How far away is Cosmic Background Radiation? [duplicate]

I've been reading about Cosmic Background Radiation (CMBR) lately. My understanding so far is that this is the "Noise"/"Flash" from the big bang. Assuming that information is correct, how far away ...
2
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4answers
248 views

Expansion of the universe and strain

From cosmological models that involve expansion of the universe, can we not say that there are ever increasing tidal forces felt by solid bodies? If so, the material in solid bodies like metal ...
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0answers
67 views

Never ending inflation

There are a lot of different models of inflation in cosmology, with plenty of different features and issues. I read that the problem with some models is that they introduce a never-ending inflation, ...
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1answer
330 views

Are dark energy and zero-point energy the same thing?

According to Quantum Mechanics is it possible that the famous "dark energy" and "zero-point energy" are the same thing that drives the accelerated expansion of the universe or maybe related to each ...
7
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1answer
202 views

Which cosmological models support the Boltzmann Brain Paradox?

In this article, they mention that the Boltzmann Brain paradox is supported by 'the best cosmological models.' However, it gives no references, and I haven't found any by a Google search. I was ...
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0answers
483 views

Making an equal amount of positive and negative energy?

I was wondering if it would be possible to create an amount of positive energy out of a vacuum, in addition to an equal amount of negative energy, thus not violating the first law of thermodynamics, ...
3
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2answers
572 views

CMBR temperature over time?

How has CMBR temperature dropped as function of time? A graph would be nice, but I'd be happy with times (age of universe) when it cooled enough to not be visible to human eye, became room temperature ...
2
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2answers
317 views

What are the relativistic effects of expanding spacetime?

This is a question I've been mulling over for a while and I'm hoping someone here can point me in the right direction. Sorry if it's a bit of a novice question. For the record, I don't fully know GR, ...
4
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2answers
348 views

Oldest population 1 star system?

While reading Stanislaw Lem's essays on advanced civilizations, I had a question: When did the earliest generation of population 1 star systems form? How much older could they reasonably be than our ...
2
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1answer
176 views

Cosmic event horizon

I had a question regarding cosmic event horizon. Let's say that a far away solar system has just crossed the cosmic event horizon due to the expansion of the universe. In that solar system let us ...
3
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3answers
328 views

How much of the universes mass is bound up in photons traveling between stars?

I was watching a Scott Manley video on youtube and he mentioned that the Sun was loosing 4 million tons of mass a second as it converts to energy. With a few trillion trillion stars also converting ...
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2answers
307 views

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 ...
3
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1answer
178 views

Baryon masses in Wetterich's new cosmology

Christoph Wetterich has put out a paper in which the universe isn't always expanding; it can be static or expanding just some of the time or even shrinking. And then there is an interaction which ...
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1answer
300 views

Can parallel universes constitute the missing mass aka dark matter?

I was pondering about this question for a while now. And with my limited knowledge of physics I can answer myself - if parallel universe theory is true and if indeed there is gravitation leakage from ...
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8answers
6k views

What is the difference between translation and rotation?

What is the difference between translation and rotation ? If this were a mathematics site, the question would be at best naive. But this is physics site, and the question must be interpreted as a ...
6
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0answers
170 views

Time diffeomorphisms breaking in inflation

I am currently working on the topic of inflation. It seems that at the stage of inflation, the universe can be described as a de Sitter space. In such a space, all spacetime diffeomorphisms are ...
2
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2answers
97 views

Cosmological models other than FRW

The FRW is a nice isotropic and symmetric metric but I think its assumptions are too many. I was wondering about alternative models. Specifically are there any prominent alternatives that have more ...
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1answer
105 views

Eddington-Weinberg relation

The numerical coincidence that triggered Dirac to postulate his Large Number Hypothesis can be summarized by expressing the proton-electron gravitational angular momentum in units $\hbar c$: ...
9
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2answers
512 views

Could spontaneous symmetry breaking happen again in our universe?

It is generally believed that $10^{-35}$ seconds after the Big Bang, the symmetry of a GUT was broken and after $10^{-12}$ seconds the electroweak force was broken: \begin{equation} \mathrm{SU(2)} ...
3
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3answers
2k 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?
4
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3answers
483 views

Dark age of universe when all fusion process ceases?

Some say we live in the golden age of the universe because there exits countless number of stars that shines in the dark universe. As the supply of gas for star formation is steadily being exhausted, ...
13
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1answer
1k views

What could explain the measurements that the Methuselah star is older than the universe?

So there has been talk in the news of a star named Methuselah that is "older than the universe". Moreover, this star happens to belong to our very own Milky Way. The article mentions that Methuselah ...
3
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1answer
398 views

Fulvio Melia's linear Universe

I would be interested in what people think of Fulvio Melia's argument for a linearly expanding Universe. I realize that the experimental evidence seems to be pointing to an accelerating Universe ...
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1answer
68 views

Newtonian approximation in cosmology

In Peeble's "Large scale structure of the Universe" the Newtonian approximation in cosmology is examined by looking at a transformation that makes the metric locally Minkowski. On pg 38 it is stated ...
16
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4answers
1k views

How would we see a near-lightspeed object emitting light?

Consider an object travelling near the speed of light relative to us (let it be a spaceship or a star), which is emitting light (consider it monochromatic resulting from a two level electronic ...
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5answers
643 views

What generates such a huge amount of gravitational attraction in a black hole?

I had been fascinated to hear that anything can be swallowed by a black hole. But what causes such a huge amount of gravitational force?
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1answer
137 views

Red shift and the expanding universe

Given that the universe is expanding because space itself is expanding, is that expansion occuring in all places and on all scales? Consider a photon emitted from a source billions of years ago. As it ...
3
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0answers
88 views

what is the radial extent of the last scattering “shell”?

At CMB recombination (z=1090), what is the radial extent of the last scattering "shell"? a) Delta(z) = .... b) Delta(comoving angular distance)= ....Mpc The WMAP first-year parameters give Delta(z) ...
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1answer
1k views

Is the total angular momentum of the universe zero? [duplicate]

I know that having a net angular momentum will contradict isotropy of the universe by preferring a specific direction. But is there any experimental data on the total angular momentum of the universe ...
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0answers
82 views

Photon temperature above Electroweak Symmetry Breaking (EWSB) transition

In discussions of the history of our Universe, photon temperature is substituted for time. As the Universe cools, phase transitions break symmetries, including electroweak symmetry. Why does it make ...
2
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0answers
75 views

Do cosmologically redshifted photons violate energy conservation? [duplicate]

I understand that, due to the Doppler effect, different frames of reference moving at different velocities relative to each other will measure different photon frequencies and hence energy. The ...
3
votes
2answers
170 views

Relation between $\Lambda$ and $\Omega_\Lambda$ in $\Lambda\mathrm{CDM}$

In minimal $\Lambda\mathrm{CDM}$, there is a parameter labeled $\Omega_\Lambda$, and current fits place it at around $\left( \Omega_\Lambda \sim 0.73\right)$. Meanwhile, $\Lambda$ enters the Einstein ...
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0answers
74 views

Closed linear cosmology implies G M / R = c^2?

I have a question about a linear FRW cosmology with $k=+1$. Assuming zero cosmological constant the first Friedmann equation can be written: $$\left(\frac{\dot R}{R}\right)^2 + ...
2
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1answer
108 views

How to calculate mass of a closed Universe?

How does one integrate the mass density over a closed Universe (a 3-sphere?) to obtain the total mass of that Universe? Is this the correct integral? $$M = R(t)^3 \rho\int_0^1 4 \pi r^2 ...
3
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1answer
429 views

What are the general relativity equations for relativistic constant acceleration?

At constant acceleration in special relativity, the time differs for a stationary observer and the astronaut. see the following article for an in-depth explanation: Relativistic Rocket However, when ...