All Questions

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0answers
29 views

Could black holes be driving the expansion? A different perspective than others that have asked [closed]

Disclaimer: I'm not a phycisist. I'm a computer science major, going to an engineer school, that's taken the required physics with calc 1 and 2. This is just an interest. Please correct anything that'...
1
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0answers
36 views

Does an expanding universe assume a space within which the expansion is happening? [duplicate]

When I read about the expanding universe and inflation I often see pictures like this: (Source: https://en.wikipedia.org/wiki/Inflation_(cosmology)) or explanation like this: For getting an idea ...
2
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0answers
58 views

Number of e-folds in phase space for the inflaton field

In José Gálvez's paper "Observational constraints on Constant Roll Inflation" it's said that, using the equation of motion for the inflaton field \begin{equation} \ddot{\phi}+3H\dot{\phi}+\frac{\text{...
5
votes
3answers
117 views

Cosmological Inflation: If Photon expands, why not other matter?

I keep repeatedly reading in many Stack Exchange and Quora questions that space of universe expands but particles (matter) don't, see e.g. this Phys.SE post. The reason given is that particles are ...
0
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1answer
68 views

How the cosmic inflation solves the horizon problem (an exact solution)?

I am reading an article, Inflation and CMBR by Charles H. Lineweaver. https://www.mso.anu.edu.au/~charley/papers/canberra.pdf (Page 5/13) He explains the inflation period as the shrinking of the ...
1
vote
2answers
61 views

$z$ (redshift) at the inflation start up time?

The $q$ parameter is at present -ve which implies accelerated expansion , hence we can approximate our universe now to be dominated by dark energy since dark energy provides exponential growth $~e^{\...
0
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0answers
16 views

How would primordial black hole forms if expansion of space overwhelms gravity? [duplicate]

I read that density is quite high in the early universe and many speculate that primordial black hole can form and would probably have a mass similar to a human ovum(egg cell). Usually black hole only ...
2
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2answers
96 views

Is inflation a possible explanation for what started the expansion?

Alan Guth stated in this article But let me start the story further back. Inflationary theory itself is a twist on the conventional Big Bang theory. The shortcoming that inflation is intended to ...
5
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1answer
72 views

Robertson-Walker metric and cosmic homogeneity

The Robertson-Walker metric is of the form $$\tag{1} ds^2 = dt^2 - a(t)^2 \Big(\frac{dr^2}{1 - kr^2} + r^2 d\theta^2 + r^2 \sin^2\theta \, d\phi^2 \Big).$$ My question is related to the $a^2(t)$ ...
1
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0answers
60 views

Could the horizon problem be explained by a finite universe instead of inflation?

Suppose the universe is finite (either closed or open with a non-trivial topology), any point could be within the horizon of another.
-1
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1answer
111 views

How can the Friedman Equation produce negative pressure or density?

As I understand it, the Friendman Equation provides the driving mechanism for Inflation.$$\frac{\ddot a}{a}=-\frac{4\pi G}{3}\left(\rho+\frac{3p}{c^2}\right)$$If $\rho$ or p is negative enough, you ...
1
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2answers
69 views

After inflation ended, did the expansion slow down to non-inflationary expansion because of gravity?

Assuming that inflation occurred, it accelerated from the previous expansion to inflationary expansion. After inflation ended, did the inflationary expansion instantly change into the previous ...
1
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2answers
61 views

Why does inflation end when the potential becomes too steep?

Inflation ends when the potential becomes too steep. Is it because kinetic energy increase when the potential becomes steeper?
0
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0answers
12 views

How do we know normal expansion without inflation is not enough to dilute the density of magnetic monopoles?

Assuming magnetic monopoles was produced in the early universe. However we don’t see any of them. Wouldn’t expansion even without inflation dilute their density enough that they are unobservable?
4
votes
2answers
165 views

Was the universe already expanding before inflation occured?

Was the universe already expanding before inflation occured or did inflation cause the universe to start off expanding? By “cause it to start off expanding” , I mean the cause of the initial ...
2
votes
2answers
87 views

What is $\tau$ and $\sigma$ in a geodesic $ψ(x) = (\tau(x),\sigma(x))$?

I'm trying to replicate the results of this paper (Eternity in 6 hours, written by Stuart Armstrong and Anders Sandberg). On pages 14-15, it discusses the path of an intergalactic probe that is ...
0
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1answer
87 views

Would eternal inflation be really eternal since quantum fluctuations are usually very rare and very small?

I’ve read somewhere that quantum fluctuations in the inflaton field are usually very rare and very small. If the effect of quantum fluctuations pushing the potential uphill is too rare and too small ...
1
vote
1answer
205 views

Alexander Vilenkin - The Universe probably had a beginning [duplicate]

Is it true that the Universe can not have an eternal past? It seems like Alexander Vilenkin tries to argue this? Does this imply that the Universe had a beginning? Link to his publication regarding ...
3
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1answer
58 views

Is stellar luminosity vs redshift calculated incorrectly?

In this paper the author, T.X. Zhang, asserts that the luminosity of distant supernovae normally is calculated incorrectly. His point (if I understand his paper) is that "luminosity" should mean ...
1
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1answer
146 views

Why do we think that the energy density of false vacuum/scalar field doesn’t dilute with the expansion?

I have a question about inflation theory. Why do we think that the energy density of false vacuum/scalar field doesn’t dilute with the expansion? Could it be that the energy density does dilute with ...
3
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0answers
47 views

Expansion of the time dimension [duplicate]

Given that the universe is expanding in the three spatial dimensions, is it possible that the time dimension is also changing over time? If the rate of the passage of time is changing over time, ...
3
votes
1answer
263 views

Why is inflation described by a scalar field?

Textbooks on cosmology describe the phenomenon of cosmic inflation in terms of the existence of a scalar field (or many scalar fields, as the answer and comments pointed out), called inflaton. Such as ...
7
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2answers
211 views

How likely are Primordial Black Holes to form in the early universe?

In the very early universe, tiny fluctuations created slight over- and under-densities in mass/energy. As far as I understand, if a region becomes sufficiently over-dense, a primordial black hole (PBH)...
1
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1answer
56 views

If the observable universe after inflation was all there was, would this form a black hole?

There have been other similar questions, but none of the answers address this point. Inflation ends after the first 10^-32 seconds. Our observable universe was the size of a grain of sand that ...
2
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1answer
55 views

The condition for cosmic acceleration

I'm currently writing a review paper on the accelerated expansion of the universe (focussing on scalar field models like quintessence) and was wondering if there is a specific name for the ...
1
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1answer
93 views

Meaning of slow-roll inflation and quintessence

I'm currently reading around the topic of quintessence and other models for dark energy and was hoping someone could explain to me what is meant by slow-roll in terms of quintessence and inflation etc....
0
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1answer
97 views

According to Newton's Law of Universal Gravitation, why isn't the universe shrinking? [duplicate]

Every point mass is attracted to each other with a force $F= \frac{GMm}{r^2}$. I know nothing about theories behind expanding universes but, why wouldn't the universe eventually shrink down to a ...
1
vote
1answer
134 views

Is it certain that accelerated space expansion is caused by negative pressure?

Pressure cause attractive gravity so we think that negative pressure should be repulsive. And I've read that negative pressure means constant energy density in an expanding universe which mean energy ...
2
votes
1answer
90 views

Does the potential $V(φ)$ of a scalar field decrease with the expansion of space?

If a scalar field (eg. inflaton field) starts with a high potential. Does the potential $V(φ)$ of the scalar field decrease with the expansion of space? If it doesn’t decrease, would it mean that ...
1
vote
1answer
128 views

Does energy conserve in de Sitter Universe?

In the de Sitter's Universe with dark energy $p=-\rho= \mathrm{const}$ it seems the total energy of the Universe does not conserved, because $E=\rho V$, and $V$ is growing with time. Am I wrong? PS ...
3
votes
1answer
71 views

How is expansion of universe seen locally? [duplicate]

Traditionally when deriving Friedmann–Lemaitre–Robertson–Walker metric, homogeneity and isotropy assumption is made. This assumption, however, holds only for large scales. To take into account some ...
0
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1answer
48 views

Why is void making CMBR lose energy as inflation speeds up?

The expansion of space is speeding up, so many voids are getting bigger. CMBR lose energy when moving through this void and then gain back the same energy as it leaves the void except in the case when ...
1
vote
1answer
255 views

Is there a relation between the gravitational constant G and the Hubble constant H?

I know there is a whole collection of question about the relation between constant X and constant Y in physics. So I should explain why this is not just a gratuitous question on some of the remaining ...
0
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0answers
64 views

Why there is needing for inflation?

Within the big bang theory, how can I conceptually understand the needing for an inflationary expansion? Why the CMB should be different than the observed one without inflation? Really the concept ...
0
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1answer
51 views

Can gradual expansion in the early universe instead of inflation creates a flat universe?

Can gradual expansion creates a smooth, uniform, and flat universe. Is it possible that the early universe was a gradual expansion instead of a very rapid and short expansion (inflation)?
3
votes
2answers
210 views

Why does an open universe expand forever even without dark energy?

I have very little basics in Physics. I have read old posts but still confused. Why would an open universe without dark energy expand forever, since there is no repulsive force(dark energy) but only ...
0
votes
1answer
186 views

Weyl curvature and Inflation of the universe

The idea of Roger Penrose is that in the first instants after the big bang, the Weyl tensor was close to zero. In his opinion, this could be a more reasonable physical explanation of the isotropy and ...
1
vote
2answers
231 views

How could inflation affect the CMB?

This is a naive question and I've just started reading inflation on my own. So the question may not be well-posed. The energy scale at which inflation took place is much higher than the scale at ...
4
votes
2answers
577 views

When did matter/energy come into existence? Before or after the CMB?

First of all thank you to anyone who replies this question, I am not a physics major, but foolishly chose to take a physics for non-science majors at my local university I am going to. I am trying ...
1
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2answers
85 views

What information about inflation could be gained from a measurement of the Cosmic Gravitational Wave Background?

I understand that inflation predicts that there is a primordial gravitational wave background due to quantum fluctuations in the gravitational field. Inflation is, of course, very important in terms ...
3
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1answer
279 views

Jim's Simple Physics Issue #i: The Big Bang Theory and Inflation

this is Jim again with another of my Simple Physics installments. This time I want to cover the much misunderstood concepts of the Big Bang theory and cosmological inflation. Like many of my other ...
1
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1answer
95 views

How do we understand time & size in the earliest moments of the universe?

1) I don't know whether this question should belong to this site: please correct me if not. In the earliest moments of the universe history probably all fundamental forces where indistinguishable: in ...
0
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2answers
114 views

Possible for certain black holes to transform into neutron stars?

Considering that space-time is expanding at an increasing rate or perhaps during times of inflation - could a black hole just above the neutron star threshold experience a decrease in density to ...
3
votes
3answers
1k views

Time dilation of distant cosmic events. What is it?

I was reading the Wikipedia's page about "tired light", where we read that any alternative explanation to the observed redshift (described by Hubble's law) should be able to overcome several ...
1
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0answers
68 views

Infinite universe - how does it expand? [closed]

I have never taken a physics course but I am fascinated. If the universe is infinite, how can it be expanding or shrinking? How can it be that black holes are so strong, nothing even light can ...
5
votes
3answers
208 views

Any proof that inflatons exist?

According to the theory of cosmic inflation, the particles that cause inflation are called inflatons. My question is how strongly a cosmologist working on inflation proves in the existence of such ...
0
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1answer
48 views

Cosmological perturbations in homogeneous and isotropic space

It is common is cosmology to study density fluctuations in the early universe. However, it is also common to assume that the background space is homogeneous and isotropic and use the FRW metric. I ...
0
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1answer
64 views

Integration when time dependent equation of state

I think it is easy but I am little confused. Can someone explain stepwise how to move from $$ \dot{\rho}=-3\frac{\dot{a}}{a}(1+w)\rho $$ to $$ \ln\frac{\rho_{re}}{\rho_{end}}=-3\int_{a_{end}}^{a_{...
2
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1answer
124 views

Cosmic inflation and space flatness

I am new here, an avid Physics reader, but have lots of questions and doubts on which I'd like to get the help of a pro physicist. Here goes some about the theory of Inflation: a) after the first ...
1
vote
1answer
2k views

What are the pieces of evidence that universe is expanding? [duplicate]

Do we have any other evidence other than red shift? Is anyone trying to disapprove inflation? Also, can red shift occur because of large distance, not only because two objects are going farther from ...