"Cosmologists are often in error, but never in doubt." -Lev Landau
I've been telling you about the Big Bang, the greatest story ever told, and the entire natural history of the Universe. Let's remind you -- historically -- of how our conception of the Universe changed as we learned more about our surroundings.
Maybe the first astronomical observation ever made was that the Sun rises in the East, passes overhead, and sets in the West. And it does this day after day, every day. It's no wonder that our first "cosmological model" of the Universe was that the Earth is stationary, and the Sun orbits around it once every day. Why not, after all? That's what we see on Earth!
But as we started making careful observations of the night sky, we discovered that the motions of planets were inconsistent with that picture. While the Sun looks like it orbits the Earth, another explanation could be that the Earth is rotating, and so the Sun would simply appear to revolve around the Earth. The better picture that superseded the old one was to put the Sun at the center, and let the Earth be just like any other planet: orbiting the Sun.
But it wasn't long before we realized our Sun wasn't unique, and that nearly all of those points of light in the night sky were also Suns, just like our own. So rather than the Sun being the center of the Universe, it became just another star -- one of billions -- in our galaxy: the Milky Way.
And amazingly, we thought that our single galaxy made up the entire Universe until the 1920s! (You can read about the discovery that changed everyone's mind here.) But now we know that our galaxy is just one of many billions in the entire Universe, which is billions of light years across, expanding, and which started with the big bang just under 14 billion years ago.
Our telescopes see farther than ever before, the amount of information we have about space is vastly greater than at any point in history, and you can find some of the most amazing images ever taken simply by using google.
But surely the current cosmological model -- the Big Bang -- isn't the final, complete story. So I thought I'd point out the three greatest problems (in my opinion, anyway) with the current cosmic model of the Universe.
1.) What happened before the Big Bang? The big bang wasn't the beginning; something had to have happened to create the Universe we have today with the properties it has. We have a word describing what must've happened before -- inflation -- but we don't know how inflation happened, or even which model of inflation is the right one! So if we want to understand where our Universe came from, we'll need to answer this question, and inflation -- while brilliant and compelling -- isn't a complete answer.
2.) What the hell is most of the matter in the Universe made of? When you look out at the Universe, you can measure the amount of mass out there (thank you, laws of gravity), and you can also measure how much normal matter is out there (thanks to light). It turns out that when we add up all the normal stuff out there -- stars, planets, gas, dust, plasma, etc. -- it's only about 15% of the total amount of matter in the Universe that gravity tells us needs to be there!
So what is this matter, and where is it? Or, quite possibly, are our laws of gravity fundamentally flawed, and is our understanding of how gravitation works on the largest scales in the Universe simply incomplete? That's the second great mystery, known as the dark matter problem.
3.) Why is the Universe's expansion speeding up? You can imagine that if things start off hot, dense, and expanding, that there are three possibilities. Gravity is going to fight that expansion, and try to pull everything back together. Maybe gravity wins, and will eventually pull things back together, causing the Universe to recollapse? Or maybe the expansion wins and gravity loses, and everything flies off into the infinite abyss? Or maybe we're in this wonderful balance between gravity and the expansion, where the expansion rate will asymptote to zero, but will never quite run away and will never reverse into a recollapsing state.
What we actually observe is none of these. It looks -- for the first few billion years of the Universe -- like it's in this perfectly balanced state. But then all of a sudden, the expansion rate picks up again, and the farthest galaxies from us accelerate away! This is known as the dark energy problem, and (while there are many speculative theories at this point) there is no compelling explanation for this phenomenon.
And those are the three biggest problems, from my point of view, facing cosmology today. What will the resolution be? Right now, nobody knows. But these are the questions, and now you can go think about them, and tell me what you think the solutions might be!
Perfect fuel for creationists...right here on scienceblogs.
You'd need to count a lot higher than 3 to point out the largest problems in creationism. :P
and, if you can count past 3, you're too smart to be a creationist.
more importantly, saying (as does creationism/ID) "if science isn't able to answer all of these questions now, it's clearly due to the actions of god (or, in id, an intelligent designer). case closed" is such an ignorant assertion you have to wonder how and why people repeat it.
Ethan, i read your earlier posts about dark matter - very good stuff. If you were a betting man (you may be) where would your bets be for where and when suggestions for answers to #2 (or any of these) will occur?
In "Don't Write off Fermilab Yet!" you wrote of the CP symmetry violation where more matter than anti-matter was created during the Big Bang and I've read else where about the constant creation of virtual particles (manifested by the Casimir effect); are more virtual matter than anti-matter particles being created? I'm guessing not, as the extra matter would show up as normal matter (or is it White-matter? - anti-Dark matter?) and be visible, on the other hand, would the non-symmetric creation of virtual photons be a source of Dark energy? Does any of this make sense?
Lastly, thanks for the series, it's a brilliant read.
How's this for ignorant speculation? -- Maybe the cosmic microwave background radiation (or something beyond it) has mass and is exerting gravitational attraction on the outer galaxies, dragging the inner ones with them.
What I've always wondered about is the current state of the accelerating universe. If all the (assumed) empty space between matter in the universe is expanding instead of at the edge (edges of the universe makes no sense to me), wouldn't that be an expanding of the universe at a static rate but look accelerating to observers at any point in it? (A bit like bubble bath where new bubbles form all the time around every bubble)
I like to learn about dinosaurs because I have a grip on the meanings of the terms, mostly, and it's fun to imagine - within the limits of what I'm familiar with. Astronomy is too tied to physics, which includes concepts I just can't be bothered to digest. I could try, but the best minds in the world are still hacking at this shit 24-7 and a lot of them are getting nowhere. I will call it wizardry and leave it to y'all.
Just explaining my ignorance and paucity of imagination before I hazard a guess, which is what you asked for: The gravity model is wrong seems the more likely of the possibilities you mentioned. The idea 85% of the mass in the Universe is HERE, all around us, in the interstices between here and the nearest star, etc., and we see NONE of it? No trace at all visible to any instrument we have? It seems insane.
But far better minds than mine are convinced by the math, and I'll lay my odds with those cats if money is involved.
I saw a computer simulation on the net not long ago which claimed to accurately depict a cube of space and the virtual particles appearing and disappearing within it. IIRC, most of the space was temporarily occupied with these particles.
Could this account for the 85% of the 'missing' mass of the Universe - the dark matter?
@Gingerbaker: The density of the virtual particles is the same everywhere, and also stays the same with passing time. Hence the virtual particles can't be the dark matter - because we already know from observations that dark matter density is *not* the same everywhere and decreases with time.
However, virtual particles ("vacuum energy") are a possible candidate for dark *energy* - the "only" problem is that the calculations of the expected density yield results which are wildly different from the observed density!
Like everyone else, I've got an insane idea. The insane idea I've got kind of covers all three...
The weirdest question up there is "what was before the big bang?" because I'm not necessarily sure it's a coherent question. Supposing that the beginning of the universe is the beginning of space AND time, the word "before" doesn't have any meaning anymore, does it?
I'm really enjoying the "Greatest Story Ever Told" series, but I have a question about the cosmic microwave background radiation. I'm trying to reconcile the expanding universe idea with the the observation of the microwave background radiation. If the radiation was created during the very early stages of the universe, when the universe was much smaller, and this radiation has traveled through space unimpeded since then, how is it that this radiation is coming towards us (so that we can see it)? Why is it not moving away from us or already well passed us so that we can't see it any longer? I'm sure that my confusion stems from a misunderstanding of what exactly the "expanding universe" means, but I have not yet found a description or explanation which makes it clear. Thanks again for a great series.
The Highest Aim of the Physicist
By Henry Rowland 11/27/1848-4/16/1901
Speech delivered to the second meeting of the American Physical Society on 10/28/1899
I hold his words with great reverence.
@Aran: Use the familiar balloon-analogy of the universe to picture this (the surface of an expanding balloon is in many ways a nice analogy for the expanding universe). On this balloon, at a time when it was still very small, lots of radiation (essentially red light) was created (due to the combination of electrons and atomic nuclei to atoms, commonly called "recombination era") - at every point of the balloon surface! That radiation then went on travelling all around the balloon, in all directions, i. e. the whole surface of the balloon was and still is filled with this radiation. The wavelength of this radiation became smaller with time, so that today, we see the whole universe filled with microwave radiation - the CMBR.
You know that sometime, years from now, we will know the answers and look back and wonder how we ever didn't know. But by then, there will be more and even grander questions.
Yup, those are the big three...and if the past is any indicator, before they get answered we'll have the re-think the whole thing over agian a few more times!
>"the Earth is stationary, and the Sun orbits around it once every day. ... But as we started making careful observations of the night sky, we discovered that the motions of planets were inconsistent with that picture."
The motions of the planets were not discovered to be inconsistent with a geocentric picture, mathematics could describe that picture as well.
In fact, with general relativity motion can be described self-consistently in any reference, there is no preferred point reference to calculate from, putting the earth at the center is a valid way of solving equations and does not lead to inconsistencies.
Those nice problems but not my primary concerns. Expressed elsewhere and I don't need to repeat here.
Here let me give version of Ethan's 3 problems.
1) What is time?
2) What is the full possibly multi-dimensional universe (not a mega-mutli-universe thing but a lean Occam's razor thing)?
3) What is the cosmic cycle (or even galactic clycle) of matter/antimatter matter/energy?
Also let me add that Bjoern continues to convince me that there is much less wiggle room for alternative theories to the big bang theories than I feel comfortable with. So I'll just have to get us to it.
@14. Thanks Bjoern that makes a lot of sense. I never really tried to put the balloon analogy together with the CMBR. I was thinking of the big bang as a point in space rather than space itself shrunk. It's much more clear now. Thanks!
The criticism of the Big Bang asks the question of what happened before. Well, if the Big Bang was the beginning of our time dimension, then the question is meaningless.
I think the author makes a common mistake of assuming that our everyday notion of causation is fundamental to existence: it does seem to be so in the universe our senses report to us.
This is an illusion brought on by the fact that events as we perceive them are gross averages of gazillions of random atomic and subatomic events--that the causation we infer from our observation is really a statistical consequence of the probabilities of these random events.
In short, if the beginning of time is part of a random event, then it does not need a prior cause.
Just so I understand this:
Matter pulls the Universe back together.
Radiation pushes the Universe apart.
Dark Matter pulls the Universe back together.
Dark Energy pushes the Universe apart.
Right? So the problem is we're getting more Dark Energy from somewhere and we don't know why?
I agree with the peeps that say it's "not necessarily ... a coherent question," as regards the thing that begat the big bang. Science's understanding of time, from what I can tell, is pretty far outside our daily experience of how time works. It is an easy out for us armchair types to say "There doesn't need to be a cause," though, because we don't need to devote much thought to that answer. Who is right? Let me know.
I think the greatest question is "Where did the original energy/matter come from?" If hte first law of thermodynamics is "energy and matter cannot be created or destroyed." Then where did it come from since it could not be created.
@Jonathan: Essentially right - only one error: radiation also pulls the Universe back together.
@mkelly: In General Relativity, and hence also in the Big Bang theory, the first law of thermodynamics does not hold - energy can be created and/or destroyed. See e.g. here:
That's the problem with the outdated and misleading habit of calling scientific descriptions "laws." No one legislated the "laws" of thermodynamics. They are nothing more nor less than a description of what we observe.
Now our observations are limited by human technology, human prejudices and limited human wisdom. They are also always subject to being upset by the next observation.
Therefore thinking of them as laws is arrogant and seriously misleading--witness the error above of thinking that this "law" demands that something "created" the mass/energy that the universe started out with and still has.
The initializing of the Big Bang was probably a quantum event, and the "laws" that pertain to quantum events are very different and counter-intuitive, if they are in fact even comprehensible in any terms except mathematics. Conservation of matter/energy still pertains at the quantum level, but not on a moment-to-moment basis. At that level one can have a free lunch on credit--you only have to pay it back sometime in the future--and the payback date can be delayed indefinitely so long as the total debt is offset by credits elsewhere.
The early universe was dominated by radiation, now it is by matter. The (initially hot) thermal background radiation has been red-shifted by expansion to ultra-cold microwave photons. Where did that energy go? I know that reference frame independence ought to make that a meaningless question but I still wonder. Does dark energy equal energy lost due to red-shifted photons?
I wish I had the time and wits to understand this better.
All of the original energy is still around--it just occupies a larger volume.
Where did that energy go?
It simply disappeared. In General Relativity, energy is in general not conserved. See the link I mentioned above (comment on April 12, 2010 2:12 PM), and also
Good links. I'll have to reread more carefully. Thanks.
Bjoern, if we are surrounded by the earliest galaxies some 7 billion light-years away, are they in turn surrounded by the dark ages, which in turn are nested within the big bang, like a series of Russian dolls? If not, why not?
A possible scenario based on the following observations and statements:
1.M-Theory and Loop Quantum Gravity Theory posit the existence of a pre-existing universe from which came our universe.
2.Our universe began at the âBig Bangâ or âBig Bounceâ (depending on which phrase one prefers).
3.Ordinary matter accounts for only 4.6% of the mass-energy density of our universe.
4.âDark Matterâ accounts for 23% of the mass-energy density of our universe and âDark Energyâ accounts for the remainder.
5.Galaxies have formed earlier than expected.
6.Some galaxies have formed in the vicinity of âDark Matterâ.
7.Our universe is expanding at an increasing rate.
8.Two families of matter will only interact gravitationally if their physics is different. [From an article by Dr. John Schwarz at Cal Tech. in "Superstrings A Theory of Everything? Edited by P.C.W. Davies and J. Brown, Cambridge University Press, ISBN 0 521 43775 X Paperback, Canto edition published in 1992]
9.Dearth of anti-particles (there is no need for a large amount of anti-particles in this scenario)
Most scenarios assume that all of the pre-existing universe formed our universe, but that need not be. There is a very interesting result if only a part forms our universe and much of the pre-existing universe remains.
There is a very large pre-existing universe interspersed with matter in a somewhat state of equilibrium. By some happening, 20% of the matter in a large volume of the pre-existing universe contracted into a very small but finite space that bounced at the big bang and expanded to form our universe. Per Martin Bojowaldâs scenario, the matter was so dense that its physics was scrambled and would not interact with the pre-existing universeâs matter except for gravitational attraction.
As our universeâs matter expanded, it encountered the pre-existing universeâs matter at various locations that providing the seed for early galaxy formation in our universe.
What is called âDark Energyâ is nothing more than the pre-existing universe matter outside of the volume and its gravitational force. Because some of the pre-existing universe matter inside the large volume contracted, the equilibrium was disturbed and a net outward gravitational force on the matter inside the volume was created thus causing the observed expansion of our universe at an increasing rate.
A new universe was born! Not only that, it was right next door! Well actually it moved in the floor above us because it's a parallel universe. When this new neighbor of ours inflated into existence it literally crossed the plane (or ceiling) of ours. Our neighbor universe had rippled through our universe, end to end, in a mere micro second. Whats more is it carried with it an abundance of anti-particles which annihilated with only regular matter (this also plays a role in explaining why there is more dark matter than regular matter). This caused a very sudden drop in the over all density of our universe. Less mass, less gravity. This is my theory for what tilted the scale in dark energy's favor, and sctarted the expansion that results in the red-shifting of all we see. I think it possible that the Cosmic Microwave Background is actually a result of these universes colliding.
That is complete hog wash, but fun to think about. Is there a current model that is similar to this idea?