The tag has no wiki summary.

learn more… | top users | synonyms

6
votes
3answers
759 views

At what speed does our universe expand?

Conceivably it expands with the speed of light. I do not know, but curious, if there is an answer. At what velocity, does our universe expand?
6
votes
2answers
1k views

A Fermi-question: Are there more stars than grains of sand?

This claim is pretty popular in the web and obviously cannot be answered by counting grains or stars. Thats what physicists call a Fermi-question. It deals with problems/questions where we dont have ...
6
votes
4answers
591 views

Is space unending?

Is the space unending, i.e it has no boundaries? If yes, how can a thing exist which is non-ending? Its impossible for me to imagine something like that. Secondly, if its not and has boundaries then ...
6
votes
3answers
190 views

If the universe is full of dark matter, why is it only 2.73 K cold?

people! I am just a physics layman, but I recently watched a documentary about the universe and it was told that the universe is full of dark matter and energy and the universe is empty, so that ...
6
votes
5answers
2k views

What is our location relative to the Big Bang?

Given what we know about space, time and the movement of galaxies, have we or can we determine what our position is in relation to the projected location of the Big Bang? I've read some introductory ...
6
votes
1answer
196 views

Curvature of the Universe imaginary?

If the curvature of the universe is zero, then $$Ω = 1$$ and the Pythagorean Theorem is correct. If instead $$Ω> 1$$ there will be a positive curvature, and if $$Ω <1$$ there will be a negative ...
6
votes
0answers
156 views

Poincare recurrence and the multiverse

In this paper Susskind claims that a stable de Sitter universe is problematic (among other things) due to the existence of Poincare recurrence, which happen because of finite entropy. I disagree that ...
5
votes
6answers
1k views

Why is the universe so organized?

If you think about the Big Bang and the flow of matter in all directions, you would think, how unorganized would this universe be? No matter how long it would take. The idea that matter or most of it ...
5
votes
2answers
143 views

Is there a consensus on the fate of our universe?

We all know that our universe is inflating from what is known as the Big Bang. However, will our universe continue to inflate at the current rate? Or after reaching a maximum size, will it collapse in ...
5
votes
6answers
4k views

How can something happen when time does not exist?

I saw this documentary hosted by Stephen Hawkins. And if I didn't get it wrong, it says that there was no time before the big bang, time was created there. So how can anything happen when there is ...
5
votes
3answers
471 views

Is the cosmic horizon related to the Big Bang event?

The Universe expands according to the Hubble's law: velocity is proportional to distance. There must be some distance, therefore, at which the velocity reaches the speed of light. This defines the ...
5
votes
4answers
917 views

How do we know the size of the universe?

Ok, from astronomical observations we can tell that the observable matter is separating - so rewind the clock about 13.7 billion years and it was all at a single point. However, how do we distinguish ...
5
votes
4answers
534 views

What does ionization of neutral Hydrogen have to do with “transparency”?

Most accounts of the early history of the Universe make some reference to (re)ionization as being the reason that the Universe becomes transparent after a period of opacity caused by the absence of ...
5
votes
2answers
253 views

Expanding universe

If we know the universe is expanding in whatever direction we look, can't we reasonably estimate where the 'center' of the Universe is? Is the rate of expansions in all directions the same?
5
votes
3answers
283 views

Is observable universe an explanation against Olbers' paradox?

First of all, let me tell you that I'm not a physicist but rather a computer scientist with a mere interest in physics at nowhere near a professional level so feel free to close this question if it ...
5
votes
1answer
113 views

Lower limit of the size of the Universe? (WMAP)

The measurement of the WMAP satellite resulted a planar geometry of the universe with a 0.4% uncertainity (http://en.wikipedia.org/wiki/Shape_of_the_universe). If there is a little deviation from the ...
5
votes
1answer
683 views

Correlation between speed, distance, mass, etc between a planet and a star (or moon and planet)?

While playing around with some little scripts, generating solar systems, I started to wonder about the correlation between the different objects. Up until now, distance, mass, size and speed are ...
5
votes
2answers
279 views

What really is Spacetime?

What is a good definition on Space, Time and the most specific topic "Spacetime"? Because someone told me that spacetime is the foundation of the entire universe? And also, Is it possible to create ...
5
votes
2answers
305 views

Light takes too long to get here

When looking at the night sky, we see lots of stars. Several places tell you that the light of those stars has traveled to many light years to reach Earth and there may be others where light has not ...
5
votes
2answers
342 views

Are we big or small?

How does the size of humans compare to the size of other objects in the universe? Are we among the relatively large or the relatively small things? My very preliminary research suggests that the ...
5
votes
4answers
553 views

If the universe is expanding, what is it expanding into?

If the universe is expanding, what is it expanding into? When the big bang happened where did it occur? When the big bang happened how did it occur? Where did the energy come from? Energy can not ...
5
votes
1answer
586 views

Alternative theories to the big bang?

Hey all, are there any theories out there on the origins (or infinite existence of) the universe beside the big bang that actually adhere to current scientific knowledge and fact?
5
votes
1answer
831 views

Is the universe finite and discrete?

Is the universe finite, both in the sense of being a closed spacetime manifold, as viewed from the macro level, but also in the sense of being fully discrete and finite in all of its intricate quantum ...
5
votes
0answers
70 views

What coordinate system is used to describe planets positions in the universe?

How are planets positions described in the space and in respect to what? For example is Sun the origo and right now at this moment Earth has [coord_X, coord_Y, coord_Z]? or maybe [lng, lat]? ...
5
votes
2answers
343 views

Does the current acceleration of universe imply that our universe is open?

Does the current acceleration of universe imply that our universe is open? If the universe is closed, from the Friedmann's equation, the acceleration of universe wouldn't be possible, would it be? (Of ...
5
votes
5answers
498 views

Do new universes form on the other side of black holes?

I have four questions about black holes and universe formations. Do new universes form on the other side of black holes? Was our own universe formed by this process? Was our big bang a black hole ...
4
votes
5answers
983 views

What is the extent of the universe?

Is there any realistic, understandable, provable (even in some extent) explanation/model for the extent of the universe? What is its shape? and Why? I mean physical explanations not philosophical ...
4
votes
2answers
546 views

Entropy of an empty universe

After watching the first episode of wonders of the solar system, one question came up which is not explained. Bryan Cox says that ultimately the universe will be devoid of matter, so not even a ...
4
votes
3answers
368 views

Why did decoherence start in the first place?

We learn that the quantum wave function $\Psi$ collapses when it interacts with a classic object (measurement). My question is: Why are there classic objects after all, how did it all start? In a ...
4
votes
1answer
391 views

Does the universe obey the holographic principle due to Stokes' theorem?

Does the universe obey the holographic principle due to Stokes' theorem? \begin{equation} \int\limits_{\partial\Omega}\omega = \int\limits_{\Omega}\mathrm{d}\omega. \end{equation} Can this ...
4
votes
3answers
206 views

What is the opposite of the Planck length?

What "large size" unit of length could be considered at the opposite end of spectrum from Planck's length? Is there a table of smallest and largest value for various physical quantities that can be ...
4
votes
2answers
1k views

Hubble time, the age of the Universe and expansion rate

The Hubble time is about 14 billion years. The estimated current age of the Universe is about 13.7 billion years. Is the reason these two time are so close (a) a coincidence, or (b) a reflection that ...
4
votes
4answers
633 views

Experimental evidence for parallel universes

My idea of physics is that it is a collection of mathematical laws relating observables. And that one can perform alot of mathematical derivations on these laws to produce new laws between ...
4
votes
2answers
132 views

How is the shape of the universe measured by scientists?

I would like to learn how scientists go about measuring the large-scale curvature of the universe to determine if the universe is closed 'i.e. spherical', flat, or open 'i.e. saddle shaped'. My ...
4
votes
1answer
279 views

Is there a center of the Universe if the Universe is finite?

Ok, let me make myself clear. I saw all the other questions related to the question, but none of them actually asks the question the way I would put it and therefore no one answers it the way I want ...
4
votes
2answers
546 views

Are all points in the universe connected?

Is it true that every point in the universe is connected or could be so theoretically? If so how is this mediated? Is it through the quantum nature of the fabric of space or is it through the ...
4
votes
5answers
143 views

Could there be more universes?

In the documentary: "Curiosity - Did God Create the Universe (on YouTube)", theoretical physicist and cosmologist Stephen Hawking states that time did not exist before the big bang. The first ...
4
votes
3answers
522 views

Can we prove that the universe is finite or infinite? [duplicate]

If I say that the universe is finite, how can you say with certain that I am wrong?
4
votes
2answers
584 views

Where does the wave function of the universe live? Please describe its home

Where does the wave function of the universe live? Please describe its home. I think this is the Hilbert space of the universe. (Greater or lesser, depending on which church you belong to.) Or maybe ...
4
votes
1answer
33 views

Open Big Bang-less universe?

This came up in discussion around a class I'm taking. For a Universe with $\Lambda$ and matter contributions to energy density (and implicitly curvature, but no radiation), can you have a universe ...
4
votes
1answer
279 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. ...
4
votes
1answer
136 views

Can the total amount of energy in the entire universe be quantified?

Classical thermodynamics leads one to believe that if energy is transferred, and the universe is heading for maximum entropy, then back extrapolating to moments before the big bang, one could find a ...
4
votes
2answers
188 views

How/why can the cosmic background radiation measurements tell us anything about the curvature of the universe?

So I've read the Wikipedia articles on WMAP and CMB in an attempt to try to understand how scientists are able to deduce the curvature of the universe from the measurements of the CMB. The Wiki ...
4
votes
2answers
232 views

Total Energy of the Universe? [duplicate]

I've heard the total energy is zero, but I've also heard it cannot be said to be zero since there's so much unknown stuff in the universe. Is that true?
4
votes
1answer
189 views

Very basic question about empty universe

I'm trying to understand the empty universe model. My first textbook (which doesn't cover the empty universe) gives the Friedmann equation as $$\left[\frac{1}{R}\frac{dR}{dt}\right]^{2}=\frac{8\pi ...
4
votes
1answer
729 views

Supermassive black holes with the density of the Universe

This question was inspired by the answer to the question "If the universe were compressed into a super massive black hole, how big would it be" Assume that we have a matter with a uniform density ...
4
votes
2answers
1k views

Physical laws prior the big bang (quantum fluctuations)

A theory among scientists says that quantum fluctuations caused the big bang and created the universe. This seems plausible to me. What I can't grasp yet is how a quantum fluctuation can even start ...
4
votes
8answers
607 views

Is relativity necessary for the existence of life?

If the universe didn't have the relativity principle, would it be able to support life? Life consists of very complicated organisms. The operation of these organisms depends on the laws of physics. ...
4
votes
0answers
212 views

Conservation of Energy in the Universe [duplicate]

Possible Duplicate: Is energy really conserved? Why can’t energy be created or destroyed? One of the laws of the universe that dazzles me the most is the law of conservation of energy. I ...
4
votes
2answers
271 views

How does the critical density decide the fate of the Universe?

Trying to teach myself cosmology. Given the Friedmann equation $$\left[\frac{1}{R}\frac{dR}{dt}\right]^{2}=\frac{8\pi G}{3}\rho-\frac{kc^{2}}{R^{2}}$$ and the critical density ...