The universe tag has no wiki summary.
32
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8answers
2k views
Is time continuous?
I was making universe simulations, and I noticed that I implemented discrete time (the only type possible on computers). By that, I mean that I had an update function, that was called many times per ...
29
votes
6answers
1k views
What is known about the topological structure of spacetime?
General relativity says that spacetime is a Lorentzian 4-manifold $M$ whose metric satisfies Einstein's field equations. I have two questions:
What topological restrictions do Einstein's equations ...
17
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5answers
2k views
Does red shift evidence necessarily imply that the universe started from a singularity?
We are taught that the universe began as a singularity - an infinitely small and infinitely dense point. At the beginning of time there was a 'Big Bang' or, more accurately, 'Inflation'.
The main ...
16
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4answers
2k views
Why is the universe so big?
The Universe is approximately 13.7 billion years old. But yet it is 80 billion light years across. Isn't this a contradiction?
15
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2answers
1k views
Why is oxygen the third most abundant element?
I was reading the article Oxygen finally spotted in space today in which it stated
Oxygen is the third most abundant element in the cosmos, after
hydrogen and helium.
Why would oxygen take ...
14
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2answers
47 views
Is there evidence of a larger universe?
While mapping out the position, distance and movement of galaxies and quasars at the limits of the observable universe, have astronomers ever observed the motion of these object due to gravitational ...
13
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4answers
327 views
Does the universe have a center?
If the big bang was the birth of everything, and the big bang was an event in the sense that it had a location and a time (time 0), wouldn't that mean that our universe has a center?
Where was the ...
12
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3answers
106 views
What is meant when it is said that the universe is homogeneous and isotropic?
It is sometimes said that the universe is homogeneous and isotropic. What is meant by each of these descriptions? Are they mutually exclusive, or does one require the other? And what implications rise ...
11
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6answers
2k views
Why is the mapped universe shaped like an hourglass?
I've watched a video from the American National History Museum entitled The Known Universe.
The video shows a continuous animation zooming out from earth to the entire known universe. It claims to ...
11
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5answers
737 views
How many times has the “stuff” in our solar system been recycled from previous stars?
Is there a cosmologist in the house? I've got a basic understanding (with some degree of error) of some simple facts:
The Universe is a little over 13 billion years old. Our galaxy is almost that ...
11
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3answers
1k views
How long will the Universe's hydrogen reserves last for?
I recently became really interested in learning about physics and cosmology, but I still know very little. Hopefully someone with more knowledge will be able to shed some light on my questions.
Here ...
10
votes
5answers
482 views
Does the amount of gravitational potential energy in the universe increase as it expands?
It seems to me that extra gravitational potential energy is created as the universe expands and the distance between massive objects such as galaxy clusters increases; this implies that energy is not ...
10
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5answers
1k views
Are we inside a black hole?
I was surprised to only recently notice that
An object of any density can be large enough to fall within its own
Schwarzschild radius.
Of course! It turns out that supermassive black holes at ...
9
votes
6answers
990 views
Does anything exist in the intergalactic space?
I am a part time physics enthusiast and I seldom wonder about the intergalactic space. First, it is my perception that all(almost all) the objects in the universe are organized in the forms of ...
9
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3answers
53 views
Seeing cosmic activity now, really means it happens millions/billions of years ago?
A Recent report about a cosmic burst 3.8 billion light years away. It is written as though it is happening now. However, my question is, if the event is 3.8 billion light years away, doesn't that mean ...
9
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2answers
249 views
Do we have an idea about the amount of matter in the universe?
Do we consider the amount of matter in the universe to be "infinite"? Or do we have an idea about "how much" there is?
9
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7answers
1k views
How many bits are needed to simulate the universe?
This is not the same as: How many bytes can the observable universe store?
The Bekenstein bound tells us how many bits of data can be stored in a space. Using this value, we can determine the ...
8
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3answers
1k views
What has been proved about the big bang, and what has not?
Ok so the universe is in constant expansion, that has been proven, right? And that means that it was smaller in the past.. But what's the smallest size we can be sure the universe has ever had?
I ...
8
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9answers
741 views
Can a universe emerge from nothing?
If the Universe is flat and the total energy of the universe can be zero (we don't know if it is, but many theorists support the idea, i.e. at BB initial conditions:
t = 0, V = 0, E = 0) then is it ...
8
votes
2answers
395 views
Is spacetime simply connected?
As I've stated in a prior question of mine, I am a mathematician with very little knowledge of Physics, and I ask here things I'm curious about/things that will help me learn.
This falls into the ...
8
votes
1answer
170 views
Dumbed-down explanation how scientists know the number of atoms in the universe?
It is often quoted that the number of atoms in the universe is 10$^{70}$ or 10$^{80}$.
How do scientists determine this number?
And how accurate is it (how strong is the supporting evidences for ...
8
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1answer
41 views
Universe Expansion
I know the galaxies are moving away from us (faster and faster) but when we say the universe is expanding, is that really only pertaining to the galaxies expanding or is the space itself expanding? I ...
8
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5answers
3k views
Stephen Hawking says universe can create itself from nothing, but how exactly?
Stephen Hawking says in his latest book The Grand Design that,
Because there is a law such as gravity, the universe can and will create itself from nothing.
Is it not circular logic? I mean, how ...
7
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4answers
489 views
How is it possible for astronomers to see something 13B light years away?
In a NPR News story from a few years back:
"A gamma-ray burst from about 13
billion light years away has become
the most distant object in the known
universe."
I'm a layman when it comes ...
7
votes
2answers
1k views
Is reality discrete at the quantum level? (…and what does it imply not only mathematically?)
On a quantum scale the smallest unit is the Planck scale, which is a discrete measure.
There several question that come to mind:
Does that mean that particles can only live in a discrete grid-like ...
7
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4answers
1k views
Did time exist before the creation of matter in the universe?
Does time stretch all the way back for infinity or was there a point when time appears to start in the universe?
I remember reading long ago somewhere that according to one theory time began shortly ...
7
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3answers
282 views
Could a universe with purely Newtonian mechanics exist?
I've heard about the fine-tuning principle and how if the fundamental constants of the Universe were changed even a slight bit, life could not exist as we know it.
However, study on this subject ...
7
votes
2answers
210 views
Is the observable region of the universe within the event horizon of a super-massive black hole?
Observations:
I have read that for a free-falling observer within the event
horizon of a black hole that all lines of sight will end at the
singularity which is black.
I also look up and see that ...
6
votes
6answers
849 views
What are the most realistic ways of high speed space propulsion?
Liquid and solid chemical fuels in rockets are very expensive and inefficient. I have heard of solar sails but what are the most realistic space travel fuels that will be used in the future to get ...
6
votes
4answers
913 views
Shape of the universe?
What is the exact shape of the universe? I know of the balloon analogy, and the bread with raisins in it. These clarify some points, like how the universe can have no centre, and how it can expand ...
6
votes
3answers
523 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
3answers
447 views
Is the total energy of the universe constant?
If total energy is conserved just transformed and never newly created, is there a sum of all energies that is constant? Why is it probably not that easy?
6
votes
4answers
173 views
Applying the Maxwell–Boltzmann statistics to astrophysical objects
Quoting Wikipedia:
In statistical mechanics, Maxwell–Boltzmann statistics describes the statistical distribution of material particles over various energy states in thermal equilibrium, when the ...
6
votes
3answers
562 views
How can we be so sure the universe has 3 dimensions?
I'm just a layman here so please bear with me if I don't get all the words or theories by correct name or whatever. I hope it will be clear enough what my question is. Here goes:
When I read articles ...
6
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4answers
486 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
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1answer
449 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?
6
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1answer
166 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 ...
5
votes
6answers
712 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
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2answers
102 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
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2answers
773 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 ...
5
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1answer
965 views
How can something finite become infinite?
How can the universe become infinite in spatial extent if it started as a singularity, wouldn't it take infinite time to expand into an infinite universe?
5
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4answers
725 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
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2answers
240 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
205 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
2answers
253 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
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4answers
1k views
Infinite universe - Jumping to pointless conclusions
I watched an episode of thee BBC Horizon series titled 'To infinity and beyond'.
In this program a number of respected physicists and mathematicians were talking about the nature of infinity and an ...
5
votes
2answers
225 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
3answers
422 views
How can a quasar be 29 billion light-years away from Earth if Big Bang happened only 13.8 billion years ago?
I was reading through the Wikipedia article on Quasars and came across the fact that the most distant Quasar is 29 Billion Light years. This is what the article exactly says
The highest redshift ...
5
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1answer
140 views
Our Universe Can't be Looped? [duplicate]
With reference to the Twin-Paradox (I am new with this), now information of who has actually aged comes from the fact that one of the twins felt some acceleration. So if universe was like a loop, and ...
4
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5answers
500 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 ...
