I often think myself into a headache, which is what I’ve done the last few days.
I was reading about Hubble’s picture of the deep field, and the ultra deep field (UDF), and other very old light from the early universe.
The UDF is supposedly a view of the universe of some 13 billion years ago… 400-800 million years after the big bang.
This got me thinking. what is the life span of light? If you look back far enough, it is supposedly theoretical that you can see just after the big bang, but this makes little sense to me. When I think of the Big bang, I think of everything in the universe exploding from a single point (the Singularity), exploding in all directions outward. If true, the light that was generated from the big bang, that leading edge of energy expanding like the exterior of a balloon being blown up forever, is gone from view. There would be no way to catch that light, since we are inside that balloon. No matter how far a telescope looked into the past, it could never see the Big bang.
Or could it?
I say it can’t, but what is the real answer? And beyond that, how close can we get to the Big Bang? If we let Hubble stare for 2 years instead of a few months, how much more would we see, and how further back in time could we see? Is there a “wall”, by which we cannot see beyond?
As for light itself, how long does it “exist”? Forever? Or is there a finite distance it can travel before it loses the ability to go beyond?
If I take an ordinary flashlight and point it at the night sky, will that beam zoom across the universe forever, and if someone had a telescope pointed at the spot at the right time, could that beam be picked up? (I understand in reality the light of the earth would obscure it, but go with me on the concepts here.)
Thanks. I have more questions, but I think this is enough to start.
From the photon’s point of view, the emission event and the absorption event are simultaneous and it exists everywhere along its path.
*Light is certainly subject to relativity. Light has zero rest mass, and relativity says that anything with zero rest mass always has to go at the speed of light along lightlike trajectories in spacetime. If you want to be anthropomorphic about it, a photon doesn’t experience the passage of time. To it, it is everywhere at once. * http://www.astro.virginia.edu/~jh8h/Foundations/quest7.html
Lots of people think this, but it’s where all the confusion starts. Stating it the way you do leads to the conclusion that the universe has a center. It doesn’t. There are no preferred spots in the universe. The entire universe expanded equally everywhere. The view from every single point is, on average, the same. Every single spot can consider itself the center.
So that makes this wrong as well. If every single spot can consider itself the center, then it can see what now exists as background microwave radiation equally well. In other words, we can see back to the first light in the universe perfectly well from earth. We see it when we see the background microwave radiation.
At about 370,000 years after the big bang the universe stopped being opaque. That’s as far back as we can see. Hubble can’t look for this, but other satellites can. That article mentions the famous Wilkinson Microwave Anisotropy Probe, but there have been others since that are even more sensitive.
Forever. Photons last until they intersect something. Every photon that existed at that 370,000 year mark and hasn’t already struck something is still out there streaming through space.
What we see in the Microwave Background is radiation that came into existence shortly after the inflation of the universe. IIRC it was once gamma radiation as the first atomic matter was forming, but has redshifted out of the visible spectrum into the microwave range.
There was a stage in the early universe where it was extremely hot and dense, there were no photons, electrons, protons, etc. just quantum particles and anti-particles, zapping in and out of existence, warping space-time with it, until a threshold was reached in energy, causing space-time to expand exponentially in an incredibly small period of time. That’s when atomic matter and radiation began to form. I’m sure I screwed that up somewhere, I’ll let a real astrophysicist/cosmologist take it from here.
And the only reason that we don’t see any light older than that is that all the photons that existed before then did, in fact, strike something. Before that time, the Universe was a lot more opaque, due to all of the matter being ionized. Then it cooled off enough for neutral atoms to form, and neutral atoms interact a lot less strongly with light, so suddenly all the light was free to propagate nearly forever.
The foam of particles and anti-particles can be said to be responsible for inflation, the brief early epoch of extremely rapid exponential growth. But inflation was over for a long, long time before we got to the point where atoms could form.
This is a fundamental error in my knowledge of the origins of the universe, then. I’ve often wondered about the “center of the universe”, and figured if the Big Bang was the way it really happened, then there must be a “center”. What you are saying is completely different than everything I’ve ever seen, heard, or read. Perhaps it’s just easier to show the Big Bang as a visual that resembles the blowing up of the death star in Star Wars. I wish the folks that make the scientific shows for the masses could tell us what you did, if that is indeed the current thinking on the subject. I think we can take it. Perhaps they are just worried it will hurt too many viewer’s heads, and they’ll change the channel.
I can sort of understand that each spot can consider itself the center of the universe, however I have a hard time understanding how we can see back to the first light in the universe. I’m trying to picture this in my head, and I can’t do it. (Where’s my damn remote?)
Here are two of the things I struggle with:
The idea of a singularity that contained all that we see (and can’t see) in the universe. To imagine just the objects in our solar system being reduced to a point that is often described as a very small point… and small is never very big. Perhaps you all that know this subject can educate me on this point. How small was the singularity? Are we talking too small to see? A softball? A earth-sized ball of compacted energy? or what?
Black holes… I don’t understand how a black hole can get to a point where it can’t seem to shove anymore matter in and has to spew out light/matter in the form of a quasar.
Ok, there are a thousand things I struggle with, but I am fascinated by the subject and the more I learn, the less I know.
A “singularity” means that there was no such thing as space before the big bang. So the singularity didn’t have any size, because there was no space for it to have size in.
And the solar system wasn’t condensed to a small point, because the solar system didn’t exist back then. Our sun and solar system are less than 5 billion years old. There are lots of stars in our galaxy that are even younger, The Pleiades for instance are only a few million years old.
No, that’s not what a black hole is. You know how if you throw a baseball up in the air, it will fall back down to earth? Well, if you could throw hard enough, you could throw the baseball right off the earth and it would sail away into space. OK, now imagine that you’re on the Sun. It’s a lot heavier with a lot higher gravity, so if you wanted to throw a baseball into space, you’d have to throw it much much harder.
Now imagine something even more massive and dense than the Sun. It takes more and more energy to throw something into space. As you look at bodies with higher and higher gravity, eventually you’d have to throw something at more than the speed of light for it to escape from the body. But since nothing can travel faster than the speed of light, that means nothing can escape the gravity of this body. That makes it a black hole. If you were standing on a black hole (bear with me) and shined a flashlight up into the sky, the light from the flashlight would sail up into space, then fall back to the black hole, just like that baseball you threw back on Earth.
This is a really common misconception, so don’t feel too bad. The actual origin of the universe is still largely in the realm of speculation and philosophy, but the Big Bang model itself tries to reach back as far as it can, from a scientific standpoint to explain a number of phenomena we see today. What we can say, is that at some point, approximately 13.7 billion years ago, all the matter and energy we see today, was compressed into an incredibly small, hot and dense state. We’re talking about energies that are outside our capabilities at the moment, and probably for a long time.
Think of the LHC as a huge microscope for peering into the realm of subatomic particles. It’s these particles that we believe formed in the first few zillionths of a second, that eventually gave rise to the separation of the four fundamental forces and eventually matter and radiation.
We know matter and energy are really the same thing, so to see subatomic particles, like bosons, leptons, etc… we need to speed up normal elemental matter (like protons) to as close to the speed of light as possible to create enough kinetic energy, smashing them together, and see what subatomic particles fly out. The faster you smash 'em, the hard they hit each other, the deeper we can look into the subatomic world, and the closer we can bring our understanding as to what the laws of physics were like at the beginning of time.
So, Around 377,000 years ago, all the photons were being thrown around by positively charged protons, negatively charged electrons (ions), filling the entire universe with a plasma of “opaque” or glowing radiation. Once the universe cooled off enough, the protons and electrons finally were able to neutralize and form Hydrogen and Helium releasing all those photons, free to fly away toward infinity… that’s what we see when looking at the Cosmic Microwave Backround, the very photons that were set free all those billions of years ago.
Remember that matter and energy are the same thing, really (E=Mc[sup]2[/sup]). When the universe was compressed so tight, there were no atoms or elementary particles… just points (and anti-points) of extremely high energy. Perhaps they’re singularities in and of themselves?
There are black holes, and then there are quasars (probably huge black holes at the center of very distant galaxies) which have hot, bright accretions discs, etc. Check out the article in Wikipedia.
I get what you are saying, but this makes little sense to me. Are you saying that all this time, this idea that a singularity being just a small point (or baseball size or whatever) is just a misunderstanding on my part? I’ve seen many of these shows on the educational channels that have implied just that. In fact, I believe how they describe it is an “infinitesimally small” point.
[aside]Just so you folks don’t think I’m a complete idiot, I’m well aware that our solar system wasn’t condensed inside a small point, or came from the big bang. I was using this as an example of just a drop of matter in the vastness of space that everyone is familiar with. To imagine all the objects in our solar system mashed down into a tiny “singularity” is hard enough to imagine… forget about understanding how the entire universe was reduced to a singularity. Lemur866 just misread what I wrote (or misinterpreted). No biggie.[/aside]
But is this true? I always thought that the speed of light was a cosmic constant… the upper speed limit for our universe. And yet, a black hole seems to imply that light (or energy/matter/photons/whatever) is travelling faster than the speed of light to escape the grip of the black hole and escape into space.
That’s what takes some getting used to. We’re used to thinking of stuff having volume, and taking up space. As it turns out, the deeper we look into this stuff, there really nothing there, there. At the moment of the Big Bang, it was an infinitesimally small (if not a singularity) dot of mind-blowingly high density and energy. No “stuff” yet, as we know it today.
It is a constant. But the gravitational forces of a black hole create an escape velocity that surpasses the speed of light (the event horizon). Which is why light cannot escape. However, stuff getting sucked into a blackhole, outside of its event horizon can form an accretion disk and create bright sources of light/radiation. (Then there’s a quantum effect at play which does let a black hole emit thermal radiation, known as Hawking radiation and will lead to black hole evaporation.)
All the modern science books cover this. I really like Brian Greene, a working physicist with a very readable style. His latest is The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos, which covers an amazing amount of territory. You can also search here, because there are dozens of recent threads that talk about the universe not having a center and what that implies.
Look up at the night sky. You see stars and black space between them. All the stars’ light comes from the past, a minimum of 4 years and maybe hundreds or thousands of years. A telescope gathers light too faint for an eye to notice, and that light can be millions and billions of years old. Other instruments gather photos from other parts of the spectrum, including microwaves. Those photons can be the oldest of all, 13.7 billion years old. But the concept is identical. As we move farther and farther away, each sphere is older and contains photons that were emitted earlier in the universe’s history. Eventually we get to a sphere that is as far as the light has traveled since the beginning.
The mental leap that needs to be made is a tough one, moving from us being the center to understanding that every point in the universe, since it was in contact at the beginning can think of itself as the center point, since the same series of expanding spheres can be drawn around any point anywhere. The usual metaphor is to imagine spots on the outside of a balloon. Any spot can think of itself as the center. None is any different from any other. It’s not obvious how that translates into three dimensions, but it does.
Greene’s book discusses ways universes can form. He also talks about the concept of inflation, that the universe expanded 10[sup]30[/sup] times in 10[sup]-35[/sup] seconds. Amazing but seemingly true.
SFP, instead of thinking about the early universe as very small, think of it as very dense.
As far as we can tell, the universe was infinite in extent at the moment of creation. However, because it was so dense everything we can currently see – all the billions of galaxies in the observable universe – was compressed to a tiny point. However, there was much more to the universe than that tiny point.
Immediately after it came into existence the universe started expanding and becoming less dense. So things that were close to each other moved farther apart. But it wasn’t an explosion. Everything everywhere was getting less dense at the same time.
For a few hundred thousand years the universe was filled with expanding clouds of superheated plasma. These clouds were so hot and thick they were opaque to light. Eventually they dispersed enough that light could shine through them.
When we look at objects in the sky we’re looking backwards in time. The farther away a galaxy is, the longer its light takes to reach us. We see the most distant galaxies as they were over 13 billion years ago. And if we look beyond the most distant galaxies, all we see is a wall of hot plasma – that primordial cloud that I mentioned in the previous paragraph.
Its light has taken so long to reach us that we can still see what the universe looked like a few hundred thousand years after the big bang. We call the light from that wall of hot plasma the cosmic microwave background. It’s not hot anymore only because its been drastically red-shifted by its billions of years of travel through the expanding void.
In a quasar, it’s not the black hole that’s emitting all the light. It’s stuff in the close vicinity of the hole. In the process of falling in, all that stuff bumps into other stuff falling in, and gets very hot, and thus produces radiation before it falls in. Once it’s actually inside the hole, it gets dark (but there’s probably still more matter behind it that’s glowing). There’s no limit to how large a black hole can get: Just keep on feeding it, and it’ll just keep on getting larger and larger.
Black holes do radiate a little from Hawking radiation, but that’s far beyond negligible even for a star-sized black hole, and just gets even more insignificant for larger holes like those in quasars.
And incidentally, one often hears black holes described as having an escape speed greater than c, but that’s not really the right way to describe them (it does give the right value for the Schwarzschild radius, but only because the errors happen to cancel out). It’s more accurate to say that there is no path from the inside of the black hole to the outside (though there is of course a path from the outside to the inside). From inside a black hole, everything outside is in the past, and is just as impossible to reach as last Thursday.
What’s a good undergrad textbook on the subject? (What is the subject? Physics? Or cosmology?)
So… is space infinite? Or does it curve on itself like the surface of a balloon: if you shine a laser in one direction, will the light eventually hit the back of your head?
Space can be positively curved, like a balloon; negatively curved, like a saddle, or flat, like a plane. Indications are that it is flat. Even if it is finite, light can’t circle around.
I haven’t been in college for 40 years. I gave up textbooks as soon as I left. The Brian Greene book I mentioned talks about this, though.
OK, let’s go with that visual. The “universe” consists of the shrapnel thrown outward by the exploding Death Star. It’s a hollow, expanding shell. The point at which the Death Star exploded is not part of the universe, because there is no shrapnel remaining at that spot. The universe is the shell itself, and the space outside and inside of the shell is outside of the universe.
Therefore, there in no spot in the universe that can be considered the center. And every spot in the universe is receding from every other spot.
This isn’t a perfect analogy, but it may make things a little bit clearer.
Actually, the curvature of space is a different question from whether it’s finite. A positive-curvature universe must be finite, but negative or zero curvature universes can be finite or infinite. And in any case, if it’s finite, it’s possible in principle for light to circle around.
Are you saying that it’s possible in principle to have a flat universe in which light can circle around or that our flat universe is one in which light can circle around? Because I thought that the former is true but the latter is not thought to be true.