The Big Bang

Atrachasis said:

P.S. If some concepts of modern cosmology seem mind-boggling to you, read some of Greg Egan's novels. The "Orthogonal" series, for example, is a crash course in relativity with a twist (that consists of basically just flipping a sign in the space-time metric tensor…).

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Sounds interesting. From the summaries I looked at, this sounds a bit like "Flatland" taken to another level. I'll put it on my list.

I believe it's possible that that is what happened when the universe came into being.

Darth Tagnan said:

First of all, I'm sceptical that human capacity is sufficient to grasp the nature of the universe - and, as such, any and all theories will be subject to such a potential limitation.

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Then again, I am of like mind as Darth: I think our puny little brains are (currently) not capable of grasping something so all-encompassing. Maybe within a few more 10,000 years of evolution...

Atrachasis said:

Given this, it might be more helpful to simply regard our space-time not as a bubble of space expanding on some external timescale, but as a four-dimensional entity with a rather pointy end. Imagine a four-dimensional sausage, the Big Bang being one of the tips (and if current models hold, there is no tip on the other side, but the sausage just gets bigger and bigger…). Most of us perceive the direction running along the length of the sausage as time.

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Interesting insights, Atrachasis. The Universe is a sausage, I love the analogy.

wiretripped said:

Interesting insights, Atrachasis. The Universe is a sausage, I love the analogy.

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A four-dimensional sausage, that bit is important!

Come to think of it, maybe we should better think of the casing as the universe, so just imagine the sausage to be 5-dimensional, OK…? A hyper-sausage, so to speak.

Which, incidentally, answers the question of what lies beyond the universe… higher-dimensional spaces… which must be stuffed with sausage filling (scientifically classified as "baryonic dark matter", undoubtedly providing a partial explanation for the decades-old mystery of dark matter).

You'll often hear it asked "what created this, that or the other", when what you should be asking is: "Does there need to be a beginning and an end". Infinity dear boy, infinity, can you comprehend?

The answer is no because humans are mortal beings. From the day we're born to the day we die all we associate with is beginnings and endings. We start new days when time never stops, we end days when no ending has taken place. Here we are at the start of a new year the old one now at an end, but there was nothing in reality that either started or ended, we merely marked the passing of time, which is eternal.

There always was something and there always will be something. When religious people talk of God being eternal no-one cares, but when a scientist talks of the universe being eternal suddenly they need to ask what came before the universe, it must be God?

The Big Bang is just a scientific theory to explain how the universe, as we know it today, came into being, to explain why things are adrift in space, apparently all moving outwards from a central core (if this is what is happening) and why the universe looks very similar to shrapnel after an explosion. Etc. Mix in the nature of combustable chemicals that scientists work with on a daily basis & it's easy to see how a theory of explosions came into being.

The Big Bang Theory does not need to answer what the universe looked like or consisted of before the explosion and so does not concern itself with that query beyond wanting to know what comprised the elements needed to generate a Big Bang.

It's not even a very complicated theory to get a grasp on it's intention or idea. It is easy to imagine it could be wrong, but if it is wrong, or you can't appreciate it, does not automatically mean the answer is therefore a grand creator, it just means we don't know enough yet to make a fully accurate prognosis.

Take diseases, for example. Once upon a time they were God's wrath on the sinful. Science showed it was nothing of the kind, definitively and without question. One minute a city was being cleansed of its sinful, the next no-one died and the disease vanished, now that's what called seeing the light.

The various scientific theories that get religious folk so worked up are the ones that people use to argue against them & their theories, and are usually the ones that aren't 100% fact yet. The ones that are now 100% fact are brushed under the carpet and not brought up for debate or questioning any more. Basically: "A creator is the dude who did everything we don't know about yet".

I am an atheist and while the "big bang" might not have happened in this very exact way, it's the best we got, and so far it's also quite solid even if they are uncertainties.

However: The question is how many "layers" of uncertainty there are. With the big bang there are maybe one or two.

If you bring any relegion into the game, you have many more "why"s which you cannot explain. And at the end of this explanation path has to be how the god came into existance. And then you basically got "another" big bang theory on top of your religion, which "popped" your god(s) into existance. Just that instead of "just" having this "coming into existance" in absolute uncertainty (which with the scientific big bang is not) is followed by why the god has values as your culture, why he created humans and how he/she/them brought humans into existance.

And as lackblogger already mentioned, some concepts are hard to grasp for humans. Though he makes it sound like nobody of us can grasp any of them at all, and that infinity is the answer.

However I think thats not exactly the case. I think the real problem we have is with "nothingness" (and we have to make up some picture on what to expect after death for example).

The universe, from all we know, works in ways where people often heared the words of, but have no idea what they actually mean.

The big bang and an expanding universe are two of them. The big bang is often thought as literal explosion and stuff is being shot into all directions. But that's not really the case. Unlike with a real explosion there isn't a center and everything is being pushed away from the center. In fact in the expansion of the universe there isn't real movement at all. Just growth or stretching. If stellar objects didn't move on their own everyhing would just be further away from everything else when time passes.

Time is another thing people don't understand. Not everything is moving forward with the same speed. Time is heavily dependent on mass/gravity. The more mass is next to you, the slower time passes. The same clock will have more ticks on a space station than on earth. And at the edge of a massive object like a star or black hole time is even slower and almost comes to a stand. So if you did a vacation at the edge of a black hole and then got back to earth, everybody would have aged much more than you did.
And in that context the "event horizon" is another hard thing to grasp. It's the point of a black hole where the gravity is strong enough to not allow anything to happen anymore. The point where the strength of gravity is so strong that nothing can escape anymore, not even light (therefore black hole).

I think if you try to understand some of these concepts a bit, it also becomes much easier to imagine the big bang. And of couse once you start looking into that, you might also get interested in how elements are formed, why and how stars die, how planets are detected, why we (probably) didnt get contacted or visited by aliens yet (and on the other hand how problematic visiting other stars is / generationship problem), or why it is very likely that we will find alien life during the next 10-20 years.

If you are interested in some of these topics I can also recommend the first couple of episodes from "Cosmos" (until it goes too much into history of single people) and "Into the Universe with Stephen Hawking" (also a space docu mini series).

Space can be quite fun and interesting.

Well spoken, Kordanor. Although we probably should not be drifting into a discussion of religion vs. science, it might be worth noting that people often complain about science as robbing the world of its "magic" or whatever... but once you delve into our current scientific perception of the world, even only superficially, you cannot help but notice that the tale science is telling us about the workings of the universe all around and within us, how we came to be, and what the future may hold in store, is much more wondrous, more vivid, much more fantastic, than the images and metaphors that writers 2,000 or 3,000 years ago managed to come up with.

I have always felt that this tale, if told properly, bore the potential to inspire humans to be the best that they can be, that it even touches on moral aspects, albeit in an inspirational, not a dogmatic way.

Unfortunately, that is not the way it is told in physics classes at school.

Just remember; it is not 100% clear that things are forever expanding. The more dark matter they discover the more likely there will eventually be a collapse. Which begs the question will things repeat ?

you said:

Just remember; it is not 100% clear that things are forever expanding. The more dark matter they discover the more likely there will eventually be a collapse. Which begs the question will things repeat ?

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Provided that general relativity holds, the matter has been settled by observations for quite a while now: The amount of dark matter falls far short from that required to reverse the expansion. If there was substantially more dark matter, galaxies would cluster in very different ways than they are observed to be. Kinematics - on all scales from galaxy clusters to the disk of the Milky Way - would look very different. No way to hide that much stuff (except perhaps for the extradimensional sausage filling…), unfortunately.

So it is probably going to get cold, dark, and lonely in a while, and eventually, we will probably be huddling around the last evaporating black holes in order to catch a bit of Hawking radiation to warm our hands by…

This would have been an ideal setup for a Grimoire joke if Cleve hadn't actually released the thing…

Take this as a question and not a statement:

Given the speed of light and limitations of our technology do we really have a clear picture of the scope of the universe and number of or distance of the farthest galaxies? If not then it still beg the picture that we can't paint a full picture of the situation - correct ?
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Another way of saying that is we have observe the near galaxies and their behavior but do we have confident we understand the depth and scope of total space et all ?

Atrachasis said:

Provided that general relativity holds, the matter has been settled by observations for quite a while now: The amount of dark matter falls far short from that required to reverse the expansion. If there was substantially more dark matter, galaxies would cluster in very different ways than they are observed to be. Kinematics - on all scales from galaxy clusters to the disk of the Milky Way - would look very different. No way to hide that much stuff (except perhaps for the extradimensional sausage filling…), unfortunately.

So it is probably going to get cold, dark, and lonely in a while, and eventually, we will probably be huddling around the last evaporating black holes in order to catch a bit of Hawking radiation to warm our hands by…

This would have been an ideal setup for a Grimoire joke if Cleve hadn't actually released the thing…

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you said:

Take this as a question and not a statement:

Given the speed of light and limitations of our technology do we really have a clear picture of the scope of the universe and number of or distance of the farthest galaxies? If not then it still beg the picture that we can't paint a full picture of the situation - correct ?
-
Another way of saying that is we have observe the near galaxies and their behavior but do we have confident we understand the depth and scope of total space et all ?

Click to expand...

Good question! The largest structures that we know of in the universe are galaxy superclusters. They are up to a few hundred million light years across (imagine that these structures might have emerged from a pre-inflationary energy fluctuation the size of an atom…), comprising tens of thousands of galaxies like our Milky Way, each brimming with billions of stars like our sun…

However, we can actually see a lot further than that. The universe is presumed to be around 13 billion years old (IF General Relativity holds…), so we can actually survey it for billions of light years in every direction (although we must account for the fact that we are seeing snapshots of earlier and earlier times as we observe the more distant structures). Much further in any case than the size of a supercluster. And on those gigantic scales, the universe appears fairly homogeneous and isotropic… like, say, a pebble beach. There are small pebbles, larger pebbles, some sand in between… but if you look at it from sufficiently far away, it appears smooth. There is a "maximum pebble size", and it is much smaller than the beach. That suggests that it is actually permissible to use fairly simple equations and assume an "average density" of the cosmos to calculate its evolution as a whole. Which is exactly what cosmologists do. The math for that is surprisingly trivial.

Of course, what might be going on beyond the horizon that we can observe is anybody's guess. But if even light from such regions has not reached us, there is no other way they could possibly have affected our little cosmic bubble. Even gravity only operates at the speed of light. There are theories that the universe we observe was originally just a "bump" in a much larger space-time continuum that was blown up by inflation, so the "meta-universe", so to speak, would stretch on beyond ours… but we would have no way to ever interact with it, and it would not affect the fate of our universe either.

Of course, there are other limits to our perception than just distance. The faintest of galaxies, which may contain only a few thousand stars, we can of course only observe at very short distances. Nonetheless, it is possible to do a census and extrapolate from there, which is just what I did when I was still in research, finding their number to increase rather steeply as you look for fainter and fainter ones. But to probe the mass of the structures in the universe and to get an idea of the variables that affect the future of the cosmos as a whole, such as the amount of matter there is, using the big and bright galaxies that you can see halfway across the universe as beacons is sufficient; they respond to gravity the same way everything else does.

you said:

Take this as a question and not a statement:

Given the speed of light and limitations of our technology do we really have a clear picture of the scope of the universe and number of or distance of the farthest galaxies? If not then it still beg the picture that we can't paint a full picture of the situation - correct ?
-
Another way of saying that is we have observe the near galaxies and their behavior but do we have confident we understand the depth and scope of total space et all ?

Click to expand...

I want to add something again, which might make it easier to understand.

Atrachasis already mentioned that the universe is rather "even" if you look at it in the whole picture.

But what's really important to understand is, that you do not need to measure the total size. As I mentioned before, its's not an explosion with a center.
Everything is getting distance from everything.

So…imagine this:
You have tons of marbles and staws, and you put them together like this


Imagine that this is the observable universe and you are in the middle. Not because you are in the middle of the universe but because you can see quality far in all directions.
Every marbe represents a cluster of galaxies.
You can only see 3 clusters in each direction but that doesn't matter.

Now imagine that the universe gets older. So you double the length of the straws representing the distance of the clusters. You see that every cluster gains distance to every other cluster.

Now imagine that you want to go back to the beginning. So you cut the original size in half. Thats still fine and you got a nice grid. Now cut it in half again, and again, and again. At some point you will run into a problem: There is no straw left. You would need a press to squeeze the marbles together.

And it really doesn't matter what the size of your original "obeserved universe" is. Whether you see one cluster in all directions, or 100. Because everything expands somewhat equaly you will run into the same problem regardless.

So if we have an estimate on how fast the universe expands, we can also give a good estimation on how it expanded in the past. And at which point everything is mushed together. And at that point you are already within the process of the big bang happening, before any stars or whatever formed. Without needing to know how much stuff there is.

But without knowing the total size how can you tell that there is not sufficient mass to force contraction in the future. Can they measure the rate of expansion over time ?

Kordanor said:

I want to add something again, which might make it easier to understand.

Atrachasis already mentioned that the universe is rather "even" if you look at it in the whole picture.

But what's really important to understand is, that you do not need to measure the total size.

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Yeah, the expansion over time can be measured and it's actually accellerating.

you said:

But without knowing the total size how can you tell that there is not sufficient mass to force contraction in the future. Can they measure the rate of expansion over time ?

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This is indeed possible and is done. The rate of expansion is observed directly by identifying "standard candles", i.e., supernovae that have a known brightness (there are a few complications, and there are other methods as well…). Comparing this to their apparent brightness in the sky gives you the distance. Measuring their "redshift" - a characteristic signature in their light spectra that all objects exhibit - gives you a direct measurement of the factor by which the universe has expanded during the billions of years that the light has travelled. That way, you can measure the expansion curve directly and compare it to models of the expansion.

However, there are other pieces of evidence. Most importantly, the quantity that you need in order to work out the expansion model is not so much the total mass, but rather the mass density. Therefore, it is sufficient to estimate the mass in a representative region of the universe. For example, you can look at the velocities of galaxies in galaxy clusters to measure the total mass in there, determine the ratio of total mass to starlight, and then simply multiply with the number of stars that you observe over large distances. Or you can use gravitational lensing to estimate the mass of such a structure.

Furthermore, computational astrophysicists routinely run simulations of how clusters, galaxies, and stars emerged from what was initially a rather smooth distribution of gas. The rate at which structures in the universe form, and the number ratios of small to large structures, also depend on parameters such as the mass density. If there's more mass around, it also tends to clump together more quickly and so on. This, too, can be compared to observations, for example by measuring how "clumped" galaxies are, and comparing the structures we see to these simulations, and thus certain models can be ruled out.

All these observations put together seem to paint a fairly consistent picture now. The amount of matter in the universe appears to be something like 25% of what would have been needed to eventually stop the expansion. However, the story doesn't end there - about 20 years ago, measurements of the expansion curve as described above indicated that the expansion wasn't just going on with little deceleration, but was actually even speeding up. It wasn't just that the mass density was too low - but there seemed to be a second, distinct effect coming into play.

At that time, an additional factor was re-introduced into the equations that Einstein had initially considered as a way to counteract gravity when he still believed in the universe having to be static, but which was then discarded and tacitly assumed to be zero for many decades - the "cosmological constant".

It's not really pretty, and possibly you might reproduce similar observations by altering the law of gravity, or by coming up with a different way of generating redshift signatures in distant objects - but on the other hand, the cosmological constant had actually always been there in the equations since the early 20th century, even though cosmologists had almost forgotten about it until those supernova measurements forced them to reactivate it.

But the amazing thing is how well everything comes together. It turns out that the cosmological constant appears to add up with the matter density to almost precisely 100%, which would have been required to make the universe geometrically flat - meaning that, if you find a ruler that's millions of light years long, you can measure distances using Pythagoras's theorem everywhere in the universe. A flat geometry is, incidentally, just what the theory of cosmic inflation predicts (imagine a bug standing on the surface of a balloon, and the the ballon suddenly gets inflated a million times - to the bug, the surface will look perfectly flat afterwards). However, unlike matter, the cosmological constant doesn't slow down the expansion, but accelerates it even more.