Universe Definition and 1000 Threads

Homework Statement Imagine that space (not spacetime) is actually a finite box, or in more sophisticated terms, a three-torus, of size ##L##. By this we mean that there is a coordinate system ##x^{\mu}=(t,x,y,z)## such that every point with coordinates ##(t,x,y,z)## is identified with every...

What are the mathematics behind multiple dimensions? Does mathematics allow for the existence of more than four dimensions? What allows the ability to possesses more that four dimensions, if there is already proof? I looked around, but I didn't find much by way of the answer I was looking for.

I think cosmologists use a poor choice of words when they say the universe is expanding. I don't dispute any of the science behind the "expanding" universe but I do want to dispute the appropriateness of the phrase to the science. I think people typically understand expanding to mean: getting...

I have heard popularized physics claims that the total energy of the universe may be zero. Supposedly positive mass energy plug negative field energy eg gravity cancel. How is this claim supported by science?

https://science.slashdot.org/story/16/04/23/0051211/neil-degrasse-tyson-says-its-very-likely-the-universe-is-a-simulation I am curious to see how this discussion goes.

I was just watching a video in which the famous physicist Brian Greene was interviewed about his book The Hidden Reality. During this interview, Greene said that it is a matter of significant debate among cosmologists about whether the size of the universe in finite or infinite. In Stephen...

Is there a scientific difference, if not, why bother with the block universe model given it is unintuitive?

Q: What was the average density of the universe at the time the light was emitted by the z = 6.56 galaxy? For the question we know the current average density of the universe and red-shift wavelength z = 6.56. It says to calculate the average density of the universe at the moment when light was...

I have heard before that for every 1 billion antiparticles there are 1 billion and 1 particles of normal matter. Has this been observed directly or just predicted? Have we only observed antimatter through its creation on earth?

How could it be tested that there exists true randomness in the universe? One could simply argue that the is information that we do not posses that causes the outcome of a measurement to occur, right?

What are the most widely accepted geometries of the universe?

My lecturer claimed that "reverse entropy stars" could exist in our universe. One of the examples he gave was that if you exposed some sort of detector in the direction of a hypothesized reverse entropy star, you could determine if it existed by whether it "sucked" photons out of the detector...

Is the universe now expanding? In that case, the Earth should be spinning away from the Sun right? I've heard that the moon is leaving the Earth 4cm-5cm every year If we are spinning away from the Sun, average temp. on the Earth must be decreasing Is there any scientific estimation on how...

I have a doubt on the expansion of the universe. In the recent years, it has been observed that the universe is expanding in an increasing speed. So the existence of dark energy was suggested. Actually is the speculation that the universe is expanding in an increasing speed because of dark...

When cosmologists talk of the "axis of evil" are they referring to the hemispheric power asymmetry or is that considered a different anomaly?

Laymen here. 1. From my understanding the universe is like the surface of a balloon. The universe is expanding as a balloon grows when air in being placed inside of it. Just like a surface of a balloon if you go in any direction in a straight line you will come back to the original point. Is...

What I mean by this is do there always have to be dimensions of space and time even or could they possibly not exist and still have a universe.

I'm not sure if this has already been discussed, but does our picture of the universe become less and less accurate the farther we look? By farther I mean light years away, when we concentrate our telescopes to a certain area light years away, could we be looking at a star that we aren't seeing...

In Brian Greene's book, Fabric of the Cosmos, he mentions that not enough time has passed for light from some parts of the universe to reach us. (I'm paraphrasing, but I think this is pretty much what he said, and I've heard similar statements from others). How is this possible if the universe...

A rubber ball is bouncing on a flat surface. Every time it bounces, it loses energy, while the sound of the ball hitting the surface accelerates (because the intervals of the ball impacting the surface become shorter with each bounce). The ball seems to be accelerating when, in fact, it is...

Alpha centauri A & B is about 4.5 light years away from us. That means, if one of your friend is flashing a light in there visible enough to be recognized here on Earth, it takes about 4.5 years before you would receive that signal. What are the chances that those stars, galaxies and...

Hi everyone, I am a 41 year old and a newbie and I have a few questions... 1. Is there a single global database of public domain scientific knowledge on the internet? 1.1. If not should there be? 2. Why aren't the base unit definitions for the International System Of Units based on a single...

I know that the dark era in the history of the universe is considered dark because stars had not as yet formed and so there was no light/photon production. Nevertheless, there were present those photons that we observe today as the CMB radiation. So when we refer to that period as being dark...

nikkkom submitted a new PF Insights post A Brief on the Expansion of the Universe Continue reading the Original PF Insights Post.

This question has been bugging me quite some time now. I'll start presenting my background for the problem: Fact: Photons are time-dependent oscillations of electric and magnetic fields as described by Maxwell's equations. Now, I've heard a lot of people, including professors saying that a...

At the time of Big Bang the size of Universe equal the size of an atom.The Universe has expanded and the time from the Big Bang to the present is finite.Then at the present time the size of the Universe is finite or infinite?

Is there any cosmological theory which said that the universe came from "nothing"? And what I mean by "nothing" here is nothing at all, no space, no time, no matter even no laws of physics, then the universe suddenly came to existence.

I understand that we can create entangled particles in the lab. But how many (non-locally) entangled particles (such as photons/polarisation or electrons/spin) exist in free nature?

The CMS or cosmological background permates the whole universe. It's mentioned so often in study halls and on documentaries that NOTHING can peak further back into the universe evolution beyond the CMS. Just want to hear all of your ideas on how YOU would solve this problem if you were on this...

Definite proof here:

I was wondering if there is a current hypothesis about the quantities of which matter particles were created? I'm not completely au fait with the standard model, but I've seen the picture...

I'm having trouble understanding the terms "flat geometry of the universe" and "baseline curvature of the universe." How can a 3D universe be flat?

Now before you excommunicate me, what I want to know is with all these sightings of galaxies 'within cooee' of the big bang, are they all in the same direction? How far can we see in the opposite direction? If we can see equidistant in both directions, would that not indicate that we are...

If the universe is finite then there must be some farthest object or farthest place at the extreme edge of the universe. If that is true and you are standing at that place what would you see? Would you see stars in one direction and black emptiness in the other? tex

Has the expansion of the Universe caused less clustering? If the expanding Universe is causing volume of space to increase, to me it would make sense that clustering would be impacted as a result of this.

In popular science around 70% of the total energy in the universe is dark energy. I’m a little bit confused regarding what this energy does. Does it influence all matter with a pushing force to make the expansion of the universe accelerate or does it create new space time fabric? I guess that...

A number of scientists subscribe to this theory. I read up on it, but none of the explanations I found really answered my questions. How should one attempt to envision a universe that is finite and bounded?

Does knowing the age of the universe (13.8 billion? ) say anything about size of the universe (besides the observable size)?

All of it. I make it magically disappear... I'm god, I guess. When I remove all the matter, is there anything left? Is the nothing used to be occupied by all that matter "something"? Is the space-time sans matter, still space-time?

I came up with a (not yet) practical idea about how to detect if the universe is a simulation. Presumably, a finite information simulation cannot be independent of reference frame. We could for example detect floating-point round off errors if we accelerate something very close to the speed of...

Physicists have observed the cosmic radiation background to conclude that the universe is flat (or within the margin of error of being flat). This means that the Universe contains the critical density needed to keep it flat, which is a mix of ordinary matter, dark matter and dark energy (I...

Hello Everyone! I've been reading a lot of articles about the beginning of the universe, Most of them is about Big Bang Theory. Aside from this theory, does Physicist ever considering another explanation of the existence of the Universe? Like what really makes the elementary particles of? Etc...

Is there any observation evidence for the expansion of the universe besides redshift?

I am trying to get my head around how much the volume of the universe increases per some time scale, say 1000yrs, how would one calculate this?

Let's assume that the universe will evolve over time to resemble the one predicted by heat death theorists. We're also assuming that the Big Bounce, Crunch, Rip etc. don't occur. Is it possible that while it resembles a heat death scenario, the heat death will never be total? That means: is it...

So I'm just learning about the higgs, electro-magnetic, strong and weak nuclear fields/forces. Is dark energy one of those? I understand there is as difference in the universe expanding from expansion and everything 'receding' from dark energy. I was watching a UC video on expansion, the Hubble...

Homework Statement I'm using python to calculate the age of the universe, by working out the distmodulus for a set of galaxys (using the V and I bands) then working out the distance in parsecs and then hubbles constant etc... I keep getting values that are 10s of magnitudes out which I assume...

In this thought experiment we place two 100 kg spheres of gold 10 billion light years distance from each other in an otherwise empty universe devoid of particles of any kind as far as is possible, then let them go and we disappear from the experiment leaving just those two spheres alone in the...

Dear PF Forum, Lunar Satelite orbits the moon, The moon orbits the earth, The Earth orbits the sun, I know that some of you know about this picture You might want to tell me. "No Steven, the Sun also orbits the earth" But for all practical purpose, we'd say that the earth orbits the sun. So...

According to the theory of relativity the speed of light is the cosmic speed limit which means(I think) nothing can go faster than the speed of light.Then how universe can expand faster than light itself?