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.

learn more… | top users | synonyms (1)

1
vote
1answer
72 views

Questions about how the curvature of universe is described with a single number

I often read of physics talking about "the" curvature of the universe, giving it some letter $K$, and claiming that we have seen that $K = 0$ within such and such certainty. I have three questions: ...
0
votes
2answers
68 views

Don't rulers expand at the same rate as the universe they measure? [duplicate]

If you placed a giant ruler between the sun and our nearest star for example, wouldn't the divisions on the ruler expand at the same rate as the universe - therefore maintaining a constant distance? ...
3
votes
4answers
77 views

Structure formation: Galaxies as different entities and stars as groups

Why do stars exist in groups like galaxies rather than individual stars, and why do galaxies exist as different entities without continuity and with large intergalactic spaces?
1
vote
2answers
97 views

Standing wave on a circle [closed]

Suppose that we have a standing wave on a circle. I heard that by gradually increasing the radius of the circle, the wavelength will also increase to keep the standing wave. Is it right? If yes, what'...
2
votes
2answers
76 views

Merging and Splitting of Black Holes [duplicate]

We know that two black holes can merge to form a single black hole with the release of vast amounts of energy. Can the reverse happen? That is can it happen that large energy is supplied to a black ...
0
votes
3answers
97 views

How is Space-Time produced? [closed]

The parable of the ant walking on the surface of a balloon tells us that as the balloon expands, more of surface is created, hence more place for the ant to walk. Space-Time is also in the same ...
1
vote
1answer
35 views

Is there energy density arising from expansion velocity?

I'm taking a first course in cosmology, and I'm having a conflict of intuitions when it comes to the relative motion of objects arising from the growth of the scale factor. Intuition 1: The growth of ...
0
votes
1answer
38 views

Number of e-foldings in cosmological inflationary theory

It seems that in a lot of lecture courses and notes, including mine and those online, seem to state that the number of e-foldings required is of the order of 50-60. Perhaps I'm looking in the wrong ...
0
votes
1answer
100 views

Should Anti de Sitter Universe collapse exponentially? (n0)

The de Sitter Universe is described by the Friedmann equation with $k=0$, $\rho=0$ and a positive cosmological constant $\Lambda$: $$\Big(\frac{\dot{a}}{a}\Big)^2=\frac{\Lambda c^2}{3}$$ The ...
1
vote
0answers
24 views

The angular diameter distance for a radiation dominated universe

I'm attempting to work out the angular diameter distance for a radiation dominated universe, with $k=0$ and $\Lambda=0$. I have gotten myself to this point: $$ D_{A}=\frac{1}{H_{0}(1+z)}\int^{z}_{0}...
3
votes
1answer
60 views

Is the cosmological horizon expanding faster than space itself?

I read that the rate of expansion of the universe is still a mystery. But if it's actually decelerating, wouldn't it mean that our cosmological horizon is expanding faster than space, and that one day ...
-2
votes
1answer
42 views

Does our Universe reside in the smallest particle? [closed]

I do not know where to ask this question in the first place, but I think here would be a start. Is there a possibility that if we go inside the smallest particle (which we have not discovered yet; ...
3
votes
1answer
48 views

Hydrogen in a glass of water

One of the great Sagan quotes is that we are made of star stuff - meaning the atoms in our bodies were formed from stellar nucleosynthesis. However, what about the hydrogen in a glass of water? Would ...
0
votes
0answers
32 views

black hole and depressions in space time fabric [duplicate]

black holes are considered to be formed when a very heavy mass makes a hole in space fabric.But a mass does not exert force on its own,it has been compared to a stone kept on rubber sheet and after a ...
0
votes
0answers
60 views

Scale factor of the universe

Do we impose that the scale factor $a(t)$ of the Universe is a continuous function? Or there is a physical meaning? Usually in physics we define functions to be continuous, such as the velocity of a ...
1
vote
1answer
51 views

What are the roles of pre-heating and re-heating in inflation?

During inflation, the universe is thought to have cooled by a "factor of 100,000 or so" The abstract of this paper mentions 'preheating' as playing a role in particle production. Is this 'preheating' ...
1
vote
0answers
63 views

My question is about big bang theory [closed]

Oh srry my english is weak i am only 11 years old but i love physics my question was accourding to maxwell and enistien nothing can go faster than light and if we give an object enough energy to move ...
0
votes
0answers
37 views

A question relating to the Higgs boson scalar field

Just wondering. The potential for the Higgs boson is given by: $$ V(\varphi)=\lambda(\varphi^{2}-v^{2})^{2} $$ where $v≃$ 246 GeV is the vacuum expectation value required to explain mass in the ...
1
vote
1answer
52 views

Slow-roll approximation for potential $V=\frac{1}{2}m^{2}\varphi^{2}$

I'm attempting to derive a solution to the slow-roll approximation for a scalar potential of the form $V=\frac{1}{2}m^{2}\varphi^{2}$. For the solution for $a(t)$ I will start by taking the slow-roll ...
1
vote
1answer
68 views

Cosmology: equation of motion for a scalar field in conformal time [closed]

So, I've derived the equation of motion for a scalar field in "normal" time, $t$: $$ \ddot{\phi}+3H\dot{\phi}+\frac{dV(\phi)}{d\phi} $$ Then, using the expressions for the scalar field density, $\...
0
votes
2answers
83 views

How is the universe flat?

I have real trouble visualising what is meant by the descriptor 'flat' when referred to the shape of the observable universe. Which one of the below is more accurate? a) It is flat in a 2D way, like ...
2
votes
2answers
115 views

Why doesn't the cosmological constant cause black holes to form?

The cosmological constant acts as matter with positive energy but negative pressure. Then a region $R$ should have energy $E\sim \Lambda r^3$. On the other hand, we know that the energy for a black ...
2
votes
2answers
109 views

How can we say time began at big bang?

I have seen many times scientists refer to time when Big Bang took place and space and time born out of singularity. 'Beginning of Time' is a very misleading statement,because process of beginning ...
1
vote
1answer
58 views

What are “the background equations” in cosmology?

We're currently working on perturbations within cosmology. There is something I have not heard before which has cropped up, that is: a reference to the term "the background equations". Are these just ...
2
votes
0answers
22 views

Rate of expansion of universe during big bang [duplicate]

We say big bang was the explosion not in the space but of the space itself that took place in a fraction of seconds. Speaking practically, we say the universe has no edge and is infinite. If the ...
1
vote
2answers
35 views

Redshift Versus Luminosity

I understand that there is a relation between the proper distance of a cosmic object and its "measurable" redshift, i.e. once you know the value of the redshift parameter z, then you actually know how ...
4
votes
5answers
182 views

Is the universe bounded?

As I understand it nobody can pinpoint an objective "center" of the universe nor "where" the Big Bang happened. It seems the observable universe is limited by our event horizon at some 14 billion ...
0
votes
0answers
34 views

continuum hypothesis in interstellar medium

I am reading some research papers on the model proposed to describe e.g. the interstellar medium. The interstellar medium, which is very rarefied, but its governing equations are based on the ...
7
votes
1answer
118 views

Gravity with more than one metric tensor

As weird as it sounds, yes, there are gravity theories with more than one metric tensor. This is called bimetric gravity. My question to those who have encountered bimetric gravity before: a) ...
6
votes
2answers
108 views

Scanning the universe - edit: expanding or shrinking

I know that this may sound as a very basic question, but how come that we can detect CMB radiation, light or gravitational waves from the big-bang era? Shouldn't this radiation has overtaken us a ...
1
vote
0answers
43 views

Did the universe have a temperature during inflation?

I've heard it said that inflation was not an equilibrium process. But I've also heard it said that during inflation, the temperature of the universe was much cooler than before or after. If the ...
1
vote
0answers
50 views

Quadrupole and Multipoles in Physics

I am a little confused over the notion of quadrupole and higher moments in physics in general. The first time I saw it was in electromagnetism, when we did multipole expansion to analyze higher ...
0
votes
2answers
79 views

Why relativity doesn't rule out Inflation theory? [duplicate]

Nothing can go faster than the speed of light, then how at the time of inflation space expanded faster than the speed of light? Clearly universe had already begun at the start (10−36 seconds) of ...
2
votes
2answers
68 views

Gravitational wave of Big bang? [duplicate]

Questions about the g-wave caused by the big bang: 1)was there a g-wave produced? 2) when will it reach us? 3) will it be too weak for us to detect(atleast now?)?
3
votes
3answers
160 views

Large gravitational waves

The recently detected gravitational wave at LIGO were extremely small - requiring a $1 billion interferometer to even detect its presence as it passes Earth. The gravitational constant G is small: ~$...
0
votes
1answer
52 views

What are the implications of the LIGO results in reference to our current Cosmological models?

I was looking for some explicit information on the implications of the LIGO results or probing eras prior to the or near to the Big Bang singularity. So, my question is therefore, what, if any, are ...
1
vote
1answer
66 views

Is there a truly stationary frame of reference? (part deux) [duplicate]

I wonder if the belief that there is no truly stationary frame of reference is really true. Here's my thinking, please poke holes in it and/or mock me :) As we understand it, before the big bang the ...
5
votes
3answers
228 views

How would we estimate, ahead of time, “the chances” of LIGO spotting black holes colliding in the period that it has been operating? [duplicate]

Can anyone summarize calculations that have been done about the theoretical probability of a detectable black hole collision happening in the observable universe within the time that LIGO has been ...
0
votes
0answers
48 views

Could the universe be a bubble in a larger mass?

I was reading this article which explains that Einstein originally thought about incorporating dark energy into relativity. It explains that dark energy is the thing that keeps pushing the universe ...
8
votes
1answer
323 views

Is there a “CGBR”?

The recent discovery by the LIGO made me wonder about this. We know that there exists a CMBR, Cosmic Microwave Background Radiation, a blanket of electromagnetic energy covering the universe, made by ...
1
vote
2answers
64 views

Is the expanding of the universe also a cause which can induce gravitational waves from mass? [duplicate]

Gravitational waves arises when mass is rotating in another mass'orbital, in explosions and of course in case of colliding black holes. But are they also created when mass is moving and speeding ...
1
vote
1answer
102 views

Is the cosmological time grosso modo isochrone?

Is the cosmological time grosso modo isochrone? by analogy with space isotropy. Or else do we have possibly great differences by analogy with great voids in the space. We know that it's not strictly ...
8
votes
1answer
138 views

How many galaxies could be the source of the recent LIGO detection?

The recent LIGO detection is pretty exciting, and a lot of people are asking whether there is a chance of optical detection of the black hole pair that created the signal. From a cursory reading of ...
3
votes
2answers
72 views

Does the speed of light change? [duplicate]

I know that there is a similar questions, but I think mine is a bit different. I wonder if with the expansion of the universe the speed of light changes. It seems that the speed of light is very ...
0
votes
0answers
65 views

Is it accurate to say that nothing can travel faster than $c$ in a GR context, where more space can be created?

Years ago, my brother and I had an argument where I was trying to convince him that nothing could travel faster than the speed of light. I was pursuing this in the context of Special Relativity. My ...
3
votes
1answer
80 views

Does a similar concept like centrifugal force exist for the whole universe? [duplicate]

Is it meaningful in the sense of falsificable to ask whether the whole universe (including everything known/observable: cosmic background radiation etc ..., excluding everything not directly ...
0
votes
1answer
53 views

Is there a limit to the size of black hole?

I have read answer by @John Rennie in regards to the size and density of black hole. In the last sentence he states that super supermassive black hole ...
0
votes
0answers
39 views

What is known about the physics of Planckbrane (another brane) in Randall–Sundrum model?

Randall–Sundrum model imagines that our universe is a five-dimensional anti-de Sitter space and the elementary particles except for the graviton are localized on a (3 + 1)-dimensional brane or branes. ...
0
votes
1answer
116 views

Density parameter (cosmology) as a function of redshift

Hopefully I'm missing some very basic algebra for this question. In essence, I need to derive the following: $$ \Omega=\Omega_{1}\frac{(1+z)}{1+\Omega_1{z}} $$ Now, I proceeded from the Friedmann ...
3
votes
0answers
40 views

Why do some stars actually produce “Gamma ray bursts”?

I looked it up but I haven't found any explanation as to why some stars produce them, I understand that collapsing and merging stars produce them, but my question is why is the energy concentrated in ...