Andrew Lange gives Segre Lecture on How the universe began, at UC-Berkeley.
July 8th, 2013 · Comments Off on Segre Lecture: How did the universe begin?
May 31st, 2009 · 15 Comments
Think of something with very little energy, very close to zero. The atoms would barely move at all, and this thing would be very close to the lowest temperature there is, absolute zero. As you add energy, the molecules move more (let’s assume this is happening in free space to make it simple). If you add enough, they go from being very ordered (say in a crystal lattice) to being less ordered, and they undergo a phase transition and we call that state of matter a liquid. Hotter gas breaks up into its constituent atoms. At thousands of degrees, electrons are stripped off a gas and it is called a plasma. So, we can go from solid to liquid to gas, and all the way up to a fully ionized plasma at millions of degrees, like at the center of the Sun or in its corona. Along the way, the atoms move about faster and faster.
In nuclear physics, we learn that when hydrogen atoms are moving very fast at the core of a proto-star (at temperatures above 10 million degrees) there is a strong likelihood of a nuclear reaction taking place to convert four hydrogen atoms (single protons, because it is too hot here for electrons to be bound) into a helium nucleus, with energy left over. This is called nuclear fusion, and the reaction space is accessible for temperatures over 10 million K, but nuclear fusion is extremely unlikely for a less energetic scenario. The point is, when temperatures are millions, billions or even trillions of degrees or MORE… as we believe the conditions were like during the first second of the Big Bang, there are many more things likely to occur that we would not expect to observe in our less energetic, less dense world. In accelerators, we have gotten the energy up to a TeV (trillion electron-volts), and we have been able to reproduce how fast particles were going between earlier than a billionth of the first second. Larger accelerators, like the LHC, get the energies even higher, which means we probe even earlier into that first second.
The History of the Universe
Why do we care? Well, scientists are fairly confident that in the first second the fundamental physics of the universe changed to become what it is today, in our much cooler universe. At the earliest tick on the cosmic clock, between t=0 and 10-43 seconds, it is believed that all the fundamental forces behaved the same. Why do we say this? We extrapolate back from what we do know… it was demonstrated in the 1980s that the electromagnetic and weak forces appear the same (their exchange particles appear to have the same properties, like mass and so on), when you get the temperature high enough, above a million billion degrees. This force appeared as one “electroweak” force, which is what the electroweak theory predicted. If we go back further, we have weak, strong and EM forces behaving the same – this was the GUT (grand unified theory) era. And, before that, the earliest and hottest era, with temperatures above 10-32 K, all four forces, including gravity behaved the same way. The reason for this is the behavior of the Higgs field at extremely high temperatures.
Let me back up now, and explain what the Higgs field is. (This is where I begin to call upon Brian Greene’s book, so I present it in a simple manner we can all understand!) Fields are all around us. All the time we are submerged in a sea of electromagnetic fields. From radio signals, cell phones, satellite communications, radar… there’s just about no escaping them! Electromagnetic fields are made up of photons, because they are the EM force carriers. The carriers of the weak force, the W and Z bosons, were discovered (as predicted) in the 1980s, as I mentioned above. The carrier particles of the strong force, that holds the nucleus of an atom together, is the gluon. And, even though we have yet to discover it, probably due to gravity being so much weaker than the other three forces, the graviton is believed to be the mediator of the gravitational force. And, we are quite familiar with living in a gravitational field!
Physicists suggest that there was another field, the Higgs field, mediated by the Higgs boson. During the first second, the temperatures started out so high (during the Planck era) that the Higgs field was wildly fluctuating. As things cooled below a trillion degrees, the Higgs field settled, but it settled to a nonzero value. (See page 258 of The Fabric of the Cosmos where it explains this is like sliding down the sides of a bowl, seeking the lowest point, but not being able to reach zero, because there is a small plateau in the center of the bowl.) This means the Higgs force is all around us. In fact, all of space is permeated with a “Higgs Ocean”. Scientists hope to prove this is the case by uncovering the Higgs particle with next-generation accelerators, like the LHC.
The Higgs field is where objects get their mass, or how we feel inertia. If the Higgs field is there, it explains why the zoo of subatomic particles have the masses they do. Particles that feel more resistance in the Higgs field, are measured to have a larger mass. The photon moves with no resistance, thus has zero mass. In fact, we can see this is why the speed of a photon in the vacuum is the fastest any object can go. Particles with mass MUST feel a greater resistance, and thus a slowing effect as they move through the Higgs Ocean. So, when the Big Bang happened, at first the concept of mass was meaningless. As the universe expanded and cooled, it underwent phase changes, or symmetry breaking, where the different forces “froze out” and their exchange particles “gained mass” (or, at least experienced the effects of it).
Brian Greene answers the question that may be on your mind now, “Isn’t the Higgs Ocean just another way of saying the cosmos is permeated with aether?” He says that yes, “it smacks of aether”. However, the fundamental difference is that aether was introduced as an analogy to how sound waves move through air. At the time, it was needed to explain how light moved through space, but we now know that light does not need a medium to propagate.
Greene goes on to discuss inflationary cosmology, and how it relates to where all the mass/energy making up the cosmos came from. He starts by explaining the work of Guth and Tye in the late 1970s. They believed that the Higgs field was basically fluctuating and ended on a “plateau” temporarily, providing a positive energy and a repulsive force that drove space to expand. Imagine that this field is like a spring. If you stretch it out, it wants to shrink, but if you compress it and confine it to a small volume, it wants to expand. (This is not related the how the Higgs field gives mass to particles, so we refer to it as the “Inflaton field”.) This period of inflation may have lasted only 10-35 seconds, but it drove the universe’s volume to increase by 90 orders of magnitude or more! Thus, the Big Bang theory states that at 10-38 seconds, the universe underwent a violent inflationary period, “the equivalent of blowing a single strand of DNA up to the size of the Milky Way Galaxy in a billionth of a billionth of a billionth of a blink of an eye!”
Which brings me to another amazing distinction. We often try to explain to people how The Universe had to be smaller than an atom, yet contain all the mass and energy we see today in all the galaxies and stars and planets (and we know this makes up less than 5% of the universe, when you consider dark matter and dark energy). Greene explains that when the universe was tiny, about 10-26 cm across, before it underwent the rapid inflationary period, it could have been filled with the inflaton field — and weigh a mere twenty pounds! That would have infused enough energy, through the rapid expansion, to account for the vast universe we see today! [Ref p.313 Fabric of the Cosmos, Greene] (Note: the Planck scale is as much smaller than an atom as an atom is smaller than us, and we are smaller than the entire observable universe!)
Jamming 20 pounds into a volume that tiny is still way, way beyond our present technological capabilities, so the likelihood anyone will be cooking up new universes in the lab are might slim, but it makes the creation of the universe a lot more fathomable. These are the kinds of issues we are trying to resolve with experiments like the LHC. This is what gets scientists excited! Combine that with the possibility that the Planck mission, or other missions to study the patterns of the microwave background in space (left over from the big bang), might tell us about what lies beyond the confines of our “observable universe”. This should blow your mind! Not only are we learning more about the universe, but we may learn things that we said a decade ago we “could” never know.
All of this IS fantastic, and it DOES get scientists excited, but we approach these tasks methodically and with humility. The fact that we can learn some things about the cosmos, does not delude us into thinking we are gods or that we can ever know it all. We accept that the universe is too vast and varied for us to ever fully experience, but we have bitten off a piece, and it tastes good! We no longer need to make up tales to explain our origins. We probe beneath the atom and beyond the galaxies, and we understand that it is all tied together in ways that go beyond our human intuition. We have so many scientific results that fill so much of our natural model of the cosmos, that no dogmatic human can stop our progress and return us to the day when we could put everything in human terms, anthropomorphizing even the stars themselves. We are in the realm where we must rely on science. Not because science has surpassed superstition, and become a religion unto itself, but because it works. Science takes our observations and subjects them to a methodology that removes as much human bias as possible, and lets us understand nature without creating hobgoblins. If we someday found the universe was capricious and the scientific method failed, we would have to seek something better. We do not “believe” in science with a blind and enduring faith, instead we “trust” in science, because it works. We trust that gravity will work the same tomorrow. We do not claim to know this irrefutably, and that is the point of science, as someone recently said to me, “Trust, but verify”. I think that sums up what science is. It is not a religion or a dogma, it is simply a way of thinking and approaching problems. It promises to take us on an amazing journey. After all, look how far we’ve come in 400 years since Galileo had the idea to turn a spyglass to the stars!
[Let me credit Brian Greene for doing such a good job of explaining the concept of the Higgs and inflaton fields in his book, The Fabric of the Cosmos. I highly recommend it. Much of what I am saying here, I am either relating from the astronomy courses I teach, or trying to distill out of his chapters 9-11.]