359 | Solo: Theories of Dark Energy
episode
Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas
1h 56m
1 speaker
4 chapters
transcribed 2 months ago
Transcript
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Transcript generated automatically by AI and may contain errors.
What is the main topic discussed in this episode?
Hello, everyone, and welcome to the Mindscape podcast. I'm your host, Sean Carroll. I do hope everyone listening understands that they are witnessing an historic occasion. This is the first time ever that on Mindscape we've had a two-part episode. We always just make history in interesting ways here at Mindscape. i know that in the past i've been happy to just go on at great length in individual episodes but i do think that this one kind of naturally breaks into two different parts and you know you don't have to listen to one part to get a lot out of the other one so it made sense to do it this way the overall topic of course is dark energy and the accelerating universe theories of how dark energy works what it might possibly be
So I planned out the episode and I realized I need to give background about vacuum energy and the cosmological constant, Einstein's old idea. And then, of course, that led to talking about the cosmological constant problem and the particle physics of it all. And so that made its own episode. That was the previous episode. And where we left off was we have a discovery experimentally that the universe is accelerating. We can explain that by invoking a cosmological constant, much like Einstein did years ago for a slightly different reason. But it raises questions we don't know the answers to. The two big ones are, roughly speaking, the cosmological constant problem which is why is the value so much smaller than you might expect it to be?
If you think about effective field theory, which is an enormously successful paradigm to think about all of quantum field theory and particle physics, the cosmological constant appears as a number in the effective field theory, and we have expectations for how big that number should be, and the actual number is smaller than the expectation by something like 10 to the minus 122. So that's bad, and it might be a clue to some interesting physics underlying what's going on. And it certainly wasn't what a lot of particle physicists ever expected. The other problem is, okay, if you think that you've measured the vacuum energy, the energy density in empty space itself, and that's making the universe accelerate, etc., etc.,
then why are we so lucky that the value of the vacuum energy is approximately the same as the value of the matter energy density in the universe today? By approximately the same, we mean, you know, three or four times or five times as much, something like that, but not ten to the hundred times as much. this is called the coincidence problem because the relative amounts of vacuum energy and matter density change as the universe expands the vacuum energy stays constant as an energy density the amount of matter in the universe as a density goes away as the universe expands it dilutes away to zero so if they're approximately the same order of magnitude today In the past, there was way more matter density than vacuum energy.
In the future, there'll be way more vacuum energy than matter. Why is it that we got so lucky to be born at just the right time? It almost suggests that there is some kind of anthropic, human-centered explanation for this. And maybe there is, but maybe there isn't. Maybe we can do better in terms of thinking of new physical explanations. So when the idea of the accelerating universe became established in the late 90s, early 2000s, physicists immediately said we can't simply say it's the cosmological constant and stop there that is causing the acceleration of the universe. Maybe it's something else. We should be open-minded. We were surprised once. We could be surprised again. And that led people to the idea of dynamical dark energy, something that's not quite the cosmological constant, but looks that way, is a pretty good approximation.
And that's something that we can look for experimentally, as well as writing down theoretical models for what it might be. And so that is what we're getting to today. We're going to talk about dynamical theories of what the dark energy could be.
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Chapters
4 chapters
1
What is the main topic discussed in this episode?
0:00–7:20
2
What puzzles motivate looking beyond the cosmological constant?
7:20–45:19
3
How do observations define dark energy's required properties?
45:19–1:23:27
4
How is the dark‑energy equation of state parameter w measured and interpreted?
1:23:27–1:55:45
Speakers
1 identifiedMore from Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas
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