# Tag Info

6

The curvature of the universe can be derived from the temperature fluctuations in the Cosmic Microwave Background. For a given amount of radiation, baryons, dark matter and dark energy in the universe, these temperature fluctuations can be calculated theoretically, and compared with observations, and so one searches for the values that yield the ...

1

I think there is a long list of things we know about but don't understand yet. For example, we know neutrinos have mass but we don't yet know what those masses are or even exactly how they acquire mass (but it's probably the Majorana process). There is a long list of things that we're still trying to figure out but that's not the central part of my point. ...

2

The cosmological distance to a faraway galaxy only depends on its cosmic redshift $z_\text{cos}$, i.e. the redshift caused by the expansion of the universe; it does not depend on the Doppler redshift $z_\text{dop}$ caused by the galaxy's motion within its local cluster. So we have $$\frac{a_0}{a(t)} = 1 + z_\text{cos}.$$ Now, what we observe is the total ...

7

The metric expansion of space is a fundamentally different phenomenon than the relative motion of two objects in the flat spacetime of Special Relativity: While special relativity constrains objects in the universe from moving faster than the speed of light with respect to each other, it places no theoretical constraint on changes to the scale of ...

4

Because space itself was expanding faster than the speed of light. Which is perfectly acceptable by the way with Special Relativity, because the speed of light is only a barrier for matter traveling through space. There is no such limit for the speed with which space itself can expand (or contract), as far as I know.

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It's important to appreciate that when you talk about a singularity at the Big Bang or the centre of a black hole you are really referring to a singularity in a particular mathematical model. A black hole (usually) means the Schwarzschild metric, and the equations that describe this become singular at $r = 0$. The Big bang means the FLRW metric, and the ...

4

Gravitational red shift is due to the energy of a photon in a gravitational field. So the photon needs energy to escape the gravitational force and therefore the wavelength increases (redshift). The cosmological redshift is due to the expansion of the universe and thus is not connected to a gravtiational field. The wavelength of the photon stretches like ...

0

If I understand your question right, you want to know what is so special about the red and infrared bands / filters in studying the spatial distribution of galaxies. It is true that the red and IR bands are used inreasingly much in the mapping of galaxies at high $z$. This is because at these redshifts, we often use the Lyman-Alpha line to both detect and ...

7

This is a rather lengthy answer as I tried to go a bit in depth; there is a short summary at the end. Will we see more or fewer stars with time? The short answer to this is: We see less stars with time, due to the fact that cosmic expansion is accelerating. Although what we really see at the relevant distances are galaxies; single stars are far too far ...

2

This is a really interesting question. I think the easy answer has already been given: Stars require lighter elements (Hydrogen, Helium) in order to generate energy (and hence light) from fusion. There is a finite (but thankfully enormous) amount of these lighter elements in our universe, all (in a "black boxed" sense) of which is gradually heading down ...

0

Star formation will slowly start to decrease in galaxies as the universe ages because of the conversion of gas, such as hydrogen, into heavier elements, such as carbon and iron. Essentially the universe is slowly running out of fuel. Stars have a hard time fusing heavier elements. Eventually there will be no stars left. What will remain are black dwarves, ...

-1

If the age of the universe is $1 / H_0$, then as the universe gets older $H_0$ would get smaller. So theoretically the recessional velocity of distant galaxies gets smaller and you should see more. Of course observation falsifies $H_0$ getting smaller and thus the expansion models are falsified. Meaning the universe looks roughly the same at all ...

-1

I am probably sure that you have heard about how the sun is going to become a red giant over the next few billion years and later become a incredibly dense white dwarf.This concept applies for all stars of all the universe, not that they all become white dwarfs, but that they all run out of fuel at some point move into another star phase. There are mainly 2 ...

0

Will we see more stars? The big bang blasted matter in all directions, resulting in the particles moving away from each other. These slowed down due to gravitation pull on each other. First forming small clumps due to their proximity, later with the clumps of clumps pulling together. This process keeps going recursively eventually forming stars, planets, ...

0

I red some times ago about a scenario in which the number of visible objects is becoming smaller and smaller. This is basically due to the Hubble's law: the further two objects are, the faster they move away from each other and when the speed exceed the speed of light, no news can come from them any more. If you take into account that the expansion of the ...

3

There are several question marks, here I try to analyze one by one. 3rd paragraph: yes and no. The clustering depends on the following two energies -- (absolute value of) gravitational potential of the galaxy $\phi$, and the kinetical energy of neutrinos $E_k$. The average energy of relic neutrinos $\bar{E}_k$ decreases as the universe expands. $\phi$ ...

0

To add to Stan Liou's point: No, it's not possible because we don't measure $\pi$ at all; rather, we define it in reference to Euclidean geometry, which is flat. However, Euclidean geometry probably has intuitive simplicity for humans because it closely matches the geometry of space in our environment. Here's one way of understanding explicitly what ...

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I read about Goodwin's "proof" that π = 3.20... Its BS and I know that. What I am wondering is whether his technique may have stumbled on something ( a warped space) years before Einstein. In short, no. Slightly longer, noooo.... Notably, even if Goodwin made some coherent sense (and he did not), doing this before Einstein is not a significant ...

-2

Q. Is Dark Matter evenly spread out in the universe? A. As far as we know, yes. Lawrence M. Krauss says, "If you removed all of the particles, all of the radiation,absolutely everything from space and all that remained was nothing that nothing would weigh something." http://www.youtube.com/watch?v=y4D6qY2c0Z8 What weighs something has mass. If you remove ...

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Dark matter is not evenly distributed. It clumps together under its own graviational pull. In fact, it clumps first and normal matter clumps later. Look at the 3rd image on this site http://www.astronomynotes.com/galaxy/s10.htm To see an example for dark matter distribution.

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The existence of dark matter comes primarily from gravitational evidence - in other words, we predict some behavior because of the force of gravity, but we don't observe that behavior and infer that dark matter is to blame. Thus, most predictions for dark matter come from locations in the universe where there are massive objects which we can observe - for ...

5

From the Friedmann equations, you can derive that $$\dot{R}^2 - \frac{8\pi}{3}G\rho R^2 = -k c^2,$$ where $\rho$ is the total density of the universe and $k$ is a constant that determines the shape of the universe: $k=-1,0,1$ for an open, flat and closed universe, respectively. If the universe is a hypersphere ($k=1$), then $R$ can be thought of as its ...

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There's a very common misconception that the Big Bang happened at a point like a bomb going off. It doesn't help that almost ever TV documentary on the subject represents the Big Bang in this way. Explaining what actually happened is hard without going into the Maths, but here's an explanation I gave taken from (of all places) the Science Fiction Stack ...

5

The question you are asking yourself is ill defined. The universe has no center, thus you cannot ask what is there. The important thing to realize is that a singularity (presented in the Big Bang Theory) is not a physical thing, you can't say "oh look at that singularity over there"(from that point the universe started). In fact a singularity is merely a ...

0

The simple answer to your question is that if there are other universes by virtue of the expansion we see in the universe around us, there is no way we can know anything about them directly (since the distance between them is increasing, and light can only go at light speed, so no information exchange). Now, with that said, Velenkin thought about Alan ...

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Hawking Radiation can occur in conditions influenced by more than just gravity .. temperature can also influence this - (see Bogoliubov Theory of acoustic Hawking radiation in Bose-Einstein Condensates, A. Recati, N. Pavloff, I. Carusotto and Hawking radiation in a two-component Bose-Einstein condensate, P.-É. Larré and N. Pavloff) This means that gravity ...

3

You are right that we do not know if the universe is finite or infinite in space. Cosmologists do now think that it has an infinite future because of the accelerated expansion rate due to dark energy but this does not tell us anything about the question of infinite space. To answer the question for space we first have to assume spacial homogeniety, i.e. ...

2

Here is another solution, somewhat resembling the answer from AlanSE: surround the star with mirror shell. The megastructure for this purpose, solar wrap was considered in this paper: Beech, M. (2010). A Dark Sun Rising - Its a Solar Wrap. Journal of the British Interplanetary Society, 63, 104-107. (online, but paywalled) Abstract: The structure of ...

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I just want to point out that it seems some people may be conflating 'dark matter' with 'dark energy.' Regular "normal" matter like electrons, neutrons, and the like, are estimated to make up about 5% of the matter/energy density of our universe. Dark matter, estimated to make up about 25% of the matter/energy density of the universe, is matter that has ...

0

While I can't seem to find a reference for it, I seem to remember the "double expansion" was a way to explain the paradox that the background radiation was too uniform for stars and clusters to form within the time frame allowed between the start of the expansion and now. If I remember correctly the theory was the initial cosmic background radiation spread ...

0

There's a blog post that may be of some interest to you here: http://motls.blogspot.com/2013/11/the-expansion-is-accelerating-due-to.html Basically, the universe is constantly expanding at an accelerated rate due to "negative pressure". This is better understood with the Second Friedmann Equation: $\frac{\ddot a}{a} =-\frac{4\pi G}{3} (\rho + 3p)$ The ...

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