Tonight, I am tuning in to a remote lecture presented by my great and good friends at The Secret Science Club
featuring astrophysicist Dr Katie Mack of the North Carolina State University. Appropriately enough for these seemingly eschatological times, the lecture is related to her new book: The End of Everything (Astrophysically Speaking). It'll be a nice vacation, contemplating an end which will be comfortably distant in spacetime, rather than an imminent one here in the Time of Coronavirus.
Dr Mack began her lecture with an overview of astrophysics, beginning with the famous 'Blue Marble' picture of the Earth, noting how lonely and fragile our planet looks. In about a billion years, the sun will swell, engulfing Mercury and Venus, and boiling the oceans of Earth, eventually drawing it in. She held out hope that humanity will reach out into the cosmos, perhaps finding new homes, though that will only forestall the end. The universe had a beginning and will have an end, it is gradually becoming less hospitable.
Dr Mack outlined five possible end of the universe scenarios: a big crunch, heat death, big rip, vacuum decay, and a bounce.
She continued with a picture of the Milky Way taken from the Southern Hemisphere, then for a simulation of what the galaxy looks like from the outside, she showed us a picture of the Andromeda Galaxy, with the supermassive black hole at its center. The Milky Way contains over 100 billion stars. There are many other galazies- the Hubble Deep Field is an image of a small patch of the sky. Most galaxies are moving away from us and from each other, because the universe is expanding (Andromeda is getting closer). If the universe is expanding, it must have been smaller, hotter, and denser than it is now- this is what led to the Big Bang theory. If the universe is infinite, it was a smaller infinity earlier.
When we observe light from distant galaxies, we see into the past- the light has been traveling for potentially billions of years. The Hubble Ultra Deep Field presents an image from thirteen billion years ago, about a half-billion years after the Big Bang, and we can actually see light from the hot plasma resulting from the Big Bang if we look far enough... light from the first formation of the universe. We are actually seeing the light from this event, and can map it out. Making a map of the universe using microwaves, a uniform background 'light' is apparent. There is a range of microwave wavelengths that form a Black Body curve which can help astrophysicists determine what the radiation source is. She compared this range to the light emitted by a hot poker. The background radiation is caused by the heat of the early universe. Little temperature fluctuations can be observed with more sensitive wavelength calibrations.
The distribution of matter, density, and the action of gravity can be extrapolated from this image, and computer models can be made to determine how galaxies and galaxy clusters were forming. Dr Mack then began her discussion of the end of the universe...
The Big Crunch: The universe, now expanding, stops expanding. The outword push of the Big Bang is braked by gravity, and the slowing expansion will reverse. Galaxies will collide with greater frequency than they do now. Our galaxy will collide with Andromeda in about four billion years, stars will string along, new stars will be formed, the supermassive black holes will merge, but our solar system (with its bloated sun) probably won't hit anything else. In the Big Crunch, the collisions won't kill the stars, the compressed radiation from the newly smaller, denser universe will do the job. Space will get so hot from this hard radiation that thermonuclear explosions will destroy the stars.
The Heat Death: This is thought to be more likely than the Big Crunch. This is a big freeze- heat is disordered energy. In the Heat Death, the universe will continue to expand forever. In the 1990s, astrophysicists tried to determine the deceleration parameter, whether the expansion of the universe would slow down due to gravity. They discovered that the expansion was increasing, and proposed that dark energy was driving the acceleration. Einstein's cosmological constant was a mathematical model of this 'stretchiness'. In the Heat Death model, the universe gets emptier and emptier, and in one hundred billion years, our galaxy, merged with Andromeda and other locals, will be isolated, with other galaxy clusters invisible. The stars will burn out, the black holes will evaporate, and matter will dissolve into a bit of trace heat, stray photons, and dark matter. Dr Mack noted that this is a sad story.
The Big Rip: If dark energy goes wrong, and is insufficiently explained by the Cosmological Constant (Einstein wasn't aware of other galaxies as more than a theoretical concept). Einstein didn't know that the universe was expanding, and he formulated the Cosmological Constant to explain why a static universe wasn't falling into itself due to gravity. The Cosmological Constant involves density over time- the amount of mass remains the same even as the universe expands. Matter and radiation become more diffuse, but the Cosmological Constant, the inherent stretchiness of space, remains the same. In one model phantom dark energy could cause density to increase. If this phantom dark energy can move galaxies apart, it can rip galaxies and stars apart, and can even rip smaller bodies apart, eventually ripping the universe apart. We don't know what dark energy is, and the phantom dark energy model can be determined by the Equation of State Parameter. If it is -1, the Heat Death is a good model, if less than -1, it's phantom dark energy, meaning a big rip is likely. Using the Planck microwave background information astrophysicists calculated the Equation of State Parameter, getting a value of -1.028 =/-0.032, suggesting that the Big Rip could be likely in about 188 billion years.
Vacuum Decay: The discovery of the Higgs Boson was a big driver of this model. The standard model of particle physics includes various quarks, leptons (such as the electron), gauge bosons (force carriers), and the Higgs boson. Gauge bosons are responsible for the strong nuclear force, the weak nuclear force, and electromagnetism. Masses can be calculated and the stability of physics can be determined. The Higgs field sets the rules for how physics works, but the value of the field is thought to have changed at one time in the lifespan of the universe. Our laws of physics are meta-stable, they are stable for now. Dr Mack likened it to a glass on a table, safe for now, but vulnerable to be knocked over- it would be more stable on the floor. The Vacuum State is the lowest energy state and determines the rules of physics for the universe- there are two vacuum states, a false vacuume state and a true vacuum state. If we are in a false vacuum state and 'fall into' a true vacuum state, the laws of physics would alter and the nature of particles will change. We should be safe, but we live in a quantum mechanical universe, so barriers are never really barriers- eventually quantum tunnelling will occur and particles will cross barriers. Subatomic particles are governed by uncertainty, and even the Higgs field could be affected by quantum tunnelling, creating an expanding bubble of the true vacuum state which will cause atoms to fail to hold together. This would occur at the speed of light, so you would never see it coming. Dr Mack quoted a paragraph from a 1980 paper by Sidney Coleman and Frank De Luccia which she characterized as the most beautiful example of scientific poetry:
“This is disheartening. The possibility that we are living in a false vacuum has never been a cheering one to contemplate. Vacuum decay is the ultimate ecological catastrophe; in a new vacuum there are new constants of nature; after vacuum decay, not only is life as we know it impossible, so is chemistry as we know it. “However, one could always draw stoic comfort from the possibility that perhaps in the course of time the new vacuum would sustain, if not life as we know it, at least some creatures capable of knowing joy. This possibility has now been eliminated.”
The lecture was followed by a Q&A. A question about the Big Crunch model led to a response that such a crunch might be an end, not followed by another Big Bang. A question about what lies outside the observable universe, beyond the Big Bang horizon led to the answer that the universe probably just carries on beyond what we can see- there is no sign of an edge. There was probably an inflation, an early rapid inflation, but we can't observe beyond it.
What caused the change in the Higgs field value changing? That's a quantum mechanics question, but the behavior of the field probably resulted from the cooling of the primordial universe. By measuring the Higgs field, we can come to the conclusion that it could shift again. It's a technical, complicated question, difficult to translate into layperson's terms.
Regarding dark matter, we have a good standard model for particle physics, and there are no big cracks, so there's no real data to point the way to go research-wise. We know we don't have the whole picture, but discovering what dark matter and dark energy are would open up whole new avenues of research. Why do we know so little about such prevalent components of the universe? They are invisible- dark matter only interacts with other matter gravitationally, it doesn't interact with electromagnetism (nor do neutrinos). There aren't enough neutrinos to account for dark matter though. We just don't know how to build a detector to find it. Dark matter particles might annihilate each other, creating normal matter, but that is theoretical. We can see how dark matter clumps by its gravitational effects, but we don't know what it is made of. Dark energy is even more difficult to study- it is uniformly distributed, invisible, and all it does is stretch space- it could be a property of space itself. We can look at growth of structures but that is not definite.
Some bastard in the audience asked about the revival of a theory that primordial black holes might explain dark matter- Dr Mack noted that small black holes might explain it, but there are no good models for their formation. They can't be ruled out, but it's an idea that continually falls out of favor and gets revived. She is not optimistic about the black hole model, but it keeps coming back, with different mass ranges proposed.
Kudos to Dr Mack for delivering a mind-expanding and entertaining lecture, and to Dorian and Margaret for presenting it. Thanks for presenting another fantastic Secret Science Club experience!
For a taste of the end of the universe, presented in entertaining fashion, here is a video of Dr Mack presenting this topic:
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