The updates continue. Last week we talked about dark matter, and this week we continue with its partner dark energy. Of course, they’re not really partners, unless you consider mysteriousness to be an attribute. Dark energy, that force that’s accelerating the expansion of the Universe. What have we learned?
This show was recorded Monday, 4/23/2018.
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NASA: What is Dark Energy?
Hubblesite: What is Dark Energy?
How Would Our Universe Be Different Without Dark Energy?
Ned Wright’s Cosmology Tutorial
What’s a Neutrinos
Podcast Transcription provided by GMR Transcription
Fraser: Astronomy Cast, Episode 488: Dark Energy Update. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, is the recovering Dr. Pamela Gay, the Director of Technology and Citizen Science at the Astronomical Society of the Pacific and the Director of CosmoQuest. Hey, Pamela, how you feeling?
Pamela: I’m doing so much better than last week. I – for those of you who didn’t follow my travails on Twitter, I started out with the world’s worst head cold, then developed strep throat, and then kind of wanted to die. So, I’m doing better now. My voice is back, mostly. I think all of me is at about 95 percent.
Fraser: Oh, good, good. That last 5 percent though, we won’t need it.
Pamela: I hope not.
Fraser: So, the updates continue. Last week, we talked about dark matter, and this week, we continue with its partner, dark energy. Of course, they’re not really partners, unless you consider mysteriousness to be an attribute. Dark energy, that force that’s accelerating the expansion of the universe. What have we learned?
Pamela: Put simply, it’s even weirder than we thought.
Fraser: Oh, good. So, we know less, except the things that we do know are that it’s weirder.
Fraser: Nice, and we waited ten years for this.
Fraser: Well, this will be the shortest episode of Astronomy Cast ever. So, before we go into the big update, let’s give people just the baseline, how do we know that there is a thing called dark energy, and what do we think it is, maybe?
Pamela: Well, we don’t know what it is. I’m just going to lay that out there. We have no idea what it is. How we know that it is – well, when we look at the cosmic microwave background radiation, we see evidence of a rate of expansion. When we look at baryonic acoustic wave oscillations and the early universe as traced out by the formation of large scale structure, we see evidence of a given acceleration expansion parameter.
When we look at supernovae measuring how far away galaxies are now to how far away they were at this point in the past, at that point in the past, and compare their measure distances with the rate at which they appear to be moving away from us, we then get a different expansion parameter. And the thought was, if we have a constant rate of expansion over time, then we should see a straight line in terms of distance vs. recession rate. If we had a decelerating universe, we would see that plot of distance vs. redshift have a slight curve in one direction. If we had a accelerating apart expansion, we would see a different curve, and it would be a nice single parameter curve, if everything was constant with time.
So, back in 1998, we realized well, yes. Yes, the universe is not decelerating with time, which we kind of thought was possible, not accelerating, not expanding at a constant rate, which we’d kind of hoped was what was happening because that makes the math real easy. Rather, we found Option C, which is the universe is accelerating itself apart. And we thought it was accelerating itself apart at a constant rate. There appeared to be, looking at all of this, roughly one proton of energy per cubic meter of space, and that energy was what was doing this constant pushing.
And the mysterious thing has been that no matter how we looked at measuring it, it always appeared that there was this constant amount of energy per cubic meter, and that as the universe has grown in size, each new volume added to the universe had the same exact constant amount of, we’re calling it energy.
Fraser: And so, I mean, up until say, 1998, the researchers had been doing these really, really detailed observations of the cause of microwave background the scale of the universe to try and just get at that ultimate question. We want to know, is this universe going to crunch back in on itself, is it just going to keep on coasting forever, or is it going to keep on coasting forever, forever? And so, that was the search. That was the search that various teams went in search of back in the late ‘90s, to look at those supernova, the distance to those supernovae to say, how far away is that supernova? When did it happen?
That’ll tell us how much the universe had expanded back then, and then use that to sort of chart the universe’s expansion over time. And this was the crazy discovery which you’re talking about, this idea that there is additional energy that was boosting the expansion of the universe.
Pamela: And this gets tied together with a whole lot of difference parameters that we use to describe the universe. One of them is the mass density of the universe. It turns out that if the universe has a certain significantly higher density, it goes crunch. If it has a significantly lower density, it expands forever. If it has this critical density, which we believe we’re at, it, without dark energy, will eventually come to a stop, given the fullness of time which we never actually get to.
Now, when you throw in this dark energy component, it means that in addition to having to worry about what is the mass density of the universe, you now have to worry about all this other non-mass like stuff that is contributing to how the universe behaves over time.
Fraser: Hey, everyone. Fraser here. Time again to thank our sponsor, Casper Mattresses, for generously sponsoring Astronomy Cast. The great thing about having a sponsor that’s been working with us for this long is now, it’s just about me being able to sort of tell you about the durability of the Casper mattresses that I’m using over now, years. And I’m still super happy every night, to climb into my Casper mattress, to go to sleep, to wake up. It’s sort of gone through the summer, and it doesn’t get too hot. And I’ve been through the winter, and it doesn’t get too cold. It warms up perfectly.
So, I’m still really super happy with my Casper mattress. And of course, you can get a Casper mattress as well. If you want to go to casper.com/astro, Casper will give you $50.00 towards select mattresses. Use the promo code ASTRO at checkout. Terms and conditions apply. So, once again, just go to casper.com/astro, use the promo code ASTRO at checkout, and get $50.00 towards a select mattress. Terms and conditions and apply. Thanks again, Casper.
There’s an amazing – and I can’t find the calculation. Ned Wright, who sort of – famous for looking into this kind of thing, put together this amazing graph of the different ways the universe would have expanded if that mass density of the universe was different. And it’s this enormous number, like 30 digits long. And I forget the exact number. I was looking for it quickly, but it just goes on, and on, and on, and on. And then, you get – after whatever, in the septillions or something, and then you get to the very end, a decimal.
And if it’s .8, then you would have an open universe. And if it was .2, then you would have a collapsing universe. And if it was right in the middle, then you would have the flat universe that we have. The fact that it’s that close, either way – it could be any other number. It could be ten times as big, a tenth as big. But the fact is, if it was any number that was any different, we would have a dramatically different universe than the one that we do. But dark energy is this additional sauce that’s been stirred in after the fact, that is sort of – again, we don’t really know. But it seems to have some additional factor that is just sort of continuing on where the Big Bang expansion played out.
Pamela: Yes, and it’s one of these things where – so now, we have a name. The name came about in 1998 when this whole discovery was made. There were two different supernovae groups that realized this is a thing. And since then, people have been trying to figure out, so is this thing actually energy? Is it that we don’t fully understand gravity, and gravity still needs an extra term? Long live Newtonian dynamics. Is it a matter that maybe there’s this scalar field in the background that we often call quiescence, that our entire universe is just sort of hung on, and this scalar field is what’s contributing what we perceive as dark energy?
So, there’s been a lot of well, huh, what is going on here? And as part of trying to understand in more and more detail, what exactly is going on here, we have cosmologists trying to study the problem from two very different perspectives. One set of groups is trying to measure all of the cosmological parameters based on the early universe. So, what is going on with the cosmic microwave background? What is going on with early structure formation? How do we put together all of the pieces to get from moment zero to well, the beginnings of the universe we now know?
The other team is trying to understand it from looking at the universe today and working backwards. So, these are the folks that are trying very carefully to constantly improve our distance measurements to Cepheid variable stars, to then use those Cepheid variable stars to measure distances to galaxies, to then look for supernovae in those galaxies and keep extended the distance ladder of things that we can use standard candle, these objects of known brightness, to measure the distance to. So, we’re working on trying to converge these two methods.
But unfortunately, in the process, here’s where we have an update. We have confused ourselves because it appears that you get one value for the expansion if you start today and work backwards, and you get a completely different value if you start at the cosmic microwave background and work forwards.
Fraser: So, is this another example of oh, we found stars that are older than the universe?
Pamela: It is going to be along those lines of new physics is going to be required to figure this out.
Fraser: Or, that the measurement – do they overlap in their error bars?
Fraser: They don’t, okay. So, both of them are well precise enough that the answer is –
Pamela: So, from Planck, we have at most, a Hubble value of 69. So, that’s – you take the value they want it to be, which is 67.8, you add to the error, so that this is the max allowed by their error value, and it gets you to 69. And it’s looking from Wendy Freedman’s group, it’s looking from Adam Riess’s group, that the folks who are working on trying to measure this from distance ladder studies, they’re getting around 72. And it doesn’t sound like a huge difference, but it’s a huge difference.
Fraser: It’s back to that number where it’s off by a tiny little bit. In this case, that’s a lot – 5 percent?
Fraser: So, it’s a big number. So, that’s not helpful. We should just go backwards, and do the show, and things will make a lot more sense than they do today.
Pamela: So, this is where people start just making particles up, which as we know, from past episodes, is one of my absolutely most hated things for people to do. So, here, once of the things that they’ve tried making up is something called sterile neutrinos. This is a fourth version of a neutrino, from the three that we have experimental evidence for. We have no experimental evidence for there to be sterile neutrinos. People have looked for evidence. There is no evidence of sterile neutrinos. But were sterile neutrinos to exist, they would change the required usage of energy to form particles in the early universe.
And by changing how the energy got used to form different kinds of particles, you can change what’s allowed for other things to have, and essentially just rebalance the early universe. And this slight rebalancing could explain it. So, that’s one possible explanation, if you like to make up particles.
Fraser: Which we – as you mentioned, you don’t like – but I mean, that’s how this works, Pamela. Come on. Someone goes hey, I – here’s a way. There could be this particle. And then, they tell the people at CERN to smash particles together, and hopefully, that particle will show up. Or, they look in their big neutrino detector mine in Sudbury, or IceCube down in Antarctica, and they find it. So –
Pamela: Yeah, no, they haven’t.
Fraser: Not yet, but come on, be patient. We’ve only just gone into it.
Pamela: So, the other thing that can happen, which is quite nice, is it could turn out that there’s something that doesn’t require creating a particle that’s at play. So, this is where people start doing things like well, what if it turns out that dark matter in the early universe actually does kind of, sort of, occasionally work and play well with others? One of the defining characteristics of dark matter is that it doesn’t interact via the electromagnetic force. So, this means it doesn’t interact with radiation. We can’t reflect light off of it. It doesn’t give off light. It ignores light. It is not a light loving particle.
But what if, in the early universe, when the energy density of light was much greater, when there was in general, a lot more radiation flying around because everything was smaller, and hotter, and more compact – what if then, under those circumstances, dark matter was like, I guess I’ll interact a little bit? That slight change in behavior that could have caused dark matter to interact just a little bit different could also explain this discrepancy.
Fraser: So, dark matter was messing with dark energy early on in the universe.
Fraser: Maybe. So, is that it?
Fraser: We’ve got some more updates.
Pamela: That’s not the only thing that could explain it.
Fraser: Or – sorry. This is the only difference. So, this is the new discovery that we’ve made, is that two very accurate measurements give different answers for how much is happening, and you’ve gone through two possible explanations of what it is. There are more.
Pamela: There are more, indeed. So, then, we get to the possibility that maybe the density of dark energy isn’t as constant with time as we think. So, if it turns out there is this itty-bitty little tiny parts per bazillion change over time with dark energy, that slight change with time, which would be reflected as a non-constant acceleration – so here on Earth, we have gravity at sea level is assumed to be 9.8 meters per second squared. It is a constant. You can drop things over, and over, and over, with the same atmospheric condition, with the exact same distance from the center of the Earth, and you will constantly get 9.8 meters per second squared.
Well, what if the value of dark energy was such that the rate of change for the expansion of the universe had its own rate of change?
Fraser: I don’t like where this is going.
Pamela: So, now, we have the universe is accelerating apart, and that acceleration itself has its own term of some sort.
Fraser: La, la, la, I’m not listening.
Pamela: The math gets really, really, really ugly. I don’t think any of us want to listen to this theory because I don’t know who wants to do this.
Fraser: But this is leading to the Big Rip.
Pamela: Well, it depends on the sign. It depends – I mean, it could be that it’s an ever slightly slowing acceleration.
Fraser: Well, that’s not so bad then.
Pamela: So, different things.
Fraser: But if it is a increasing one, we get this scenario in a few billion years, where the expansion is so great that it tears apart galaxies. It eventually tears apart solar systems. It eventually tears apart atoms and black holes. And there’s nothing left. And we don’t get to have those trillions and trillions of years, huddled up next to a red dwarf star, becoming spiritually enlightened. We just get torn apart at an atomic scale. See, that’s why I didn’t like where that was going. But now, I’m a little sort of more balanced about this.
Pamela: And it all depends on how much of this is happening as well. So, we have to worry about the size, which can’t be that great because otherwise we would have detected it – it varies with the size, and it also varies with the sign. So, it all comes down to who is wrong. Is it the early universe people? Is it the modern universe people? And then, there’s always the question of, do we actually know where the Cepheid variable stars are? Because if you screwed up your Cepheids, then everything goes downhill from there.
Fraser: So, hold on. So, what you’re saying is that the measurement isn’t necessarily wrong, but your measurement stick might be wrong.
Pamela: Yes. So, this is the problem of using the meter stick that the dog chewed the end off of.
Fraser: That sounds like a problem you occasionally have to deal with.
Pamela: It is, it is. This is true. So, it’s not like a dog is going to chew off the distance to a Cepheid variable star, but it’s quite possible that we had some sort of a systematic error, we had some sort of a something. And this is where folks like Adam Riess’s research team are working to find new ways to use the Hubble Space Telescope to more accurately measure the distance to Cepheids. This is why the good folks working on the Gaia Mission are working to verify with greater and greater accuracy, the distance to Cepheid variable stars.
We need to confirm where the beginning of our meter stick is, and then work on expanding out to get distances to galaxies, get distances – well, to more distant galaxies.
Fraser: So, then, where are we at with a dedicated dark energy satellite, or survey, or a ground-based instrument that is going to be the – I don’t know, the Planck of its – to do this job to really measure dark energy as best as we can from the expansion side, as opposed to the beginning side?
Pamela: Ask me after another congressional election.
Fraser: After another – didn’t you guys ask for one in the last decadal survey?
Pamela: We did, and it in fact was the highest ranked thing that we asked for. But the problem that we’re dealing with is Trump has said, no more WFIRST. Congress has said, we’ll fund you for the rest of the year. And so, while WFIRST is continuing to get built, many support projects have been canceled because of Trump’s desire to end WFIRST. So, yeah, there are ongoing cancellations taking place.
Fraser: So, what were some of them? There was the Joint Dark Energy Mission. There was the Dark Energy Survey Telescope. There is a bunch that are sort of in the works at this point.
Pamela: And so, we have down at Hobbly-Eberly Telescope, the HETDEX Project, which is ground-based. It’s undergoing early science. The Dark Energy Probe is still in the process of coming up to speed. WFIRST is at the whim of congress at the moment. Lots of different teams trying to get out ahead, before they get cancelled because Trump has said that he doesn’t think that we need to be funding dark energy.
Fraser: But there are – I mean, it’s not just a U.S. thing.
Fraser: I mean, there’s the Dark Energy Survey – is attached to the – there’s a telescope in Chile. So, there is a bunch of ground-based missions. And as you said, there is the Gaia Mission that is going to do the best job of determining the positions of stars, and hopefully, it’s going to turn up a pile of Cepheid variables as well. And there are even things like TESS which can – it has a whole – group of scientists are going to be using it for other purposes beyond just searching for exoplanets. So, there is some – sort of ideas in the works on how to get more about this data.
Do you think this will come back up again, say in the 2020 decadal survey? Is the No. 1 priority is give us as precise a measurement for dark energy as we have for the cause of microwave background?
Pamela: I’m not conceited enough to think that I can say what the No. 1 priority of the U.S. research community is going to be. But I feel confident saying that trying to figure out what dark energy is will be one of the top priorities. So, we just need to see where it lands next to astrobiology, the search for exoplanets, and all the other mysterious and weird science that’s out there. But dark matter and dark energy continue to torture us by not revealing their secrets.
Fraser: Which one would you say is better along between dark matter and dark energy? Although, I don’t even like saying that in the same sentence because they are two completely different things. Maybe they interacted, but they both need to be renamed.
Pamela: Yes. Dark matter is stuff. It’s a thing. We need to – just like we have baryonic particles, we can’t quite say that all of dark matter is non-baryonic particles, but we need some word classification to help us with this one. And dark energy, it’s not even a stuff, people. It’s part of cosmology. It’s baked into the structure of the universe.
Fraser: So, then, are there any experiments coming up, any extension research projects that you’re excited about, any key measurements that people will be able to make? When we come back around in ten years and have this conversation again, do you think that we will have made progress? What kind of progress do you think will have been made by the time we do the next update on dark energy?
Pamela: I’m really hoping that as we build more and bigger telescopes, and hopefully someday, actually launch the James Webb Space Telescope, we are able to converge our two sets of data, where we’re able to take measurements of the cosmic microwave background, take measurements of early structure formation and baryonic acoustic oscillations, take these early measurements, and converge on measurements that look at distance vs. redshift. It’s going to mean making a whole lot – higher redshift observations of supernovae.
And it’s going to be a whole lot of detailed work, but I think this kind of converging of datasets to have overlapping domain is going to be one of the things that really needs to happen. Now, I’m just one person, and that’s my personal, please make your data overlap kind of wishes. I’m sure there are people who do cosmology day in and day out, who have different wishes than mine. And luckily, they’re the ones who will be determining that part of the decadal survey.
Fraser: So, to wrap this up, man, worst update ever.
Pamela: Yes, but this is why we science.
Fraser: Dark energy, what is it? I don’t know. And it looks like we have two measurements now, and the measurements are both accurate enough to know that they’re not wrong, but they’re different, which is weird.
Fraser: We’ll see in ten years.
Pamela: Sounds good.
Fraser: I love these updates. This is a lot of fun. Thanks, Pamela.
Pamela: Thank you.
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Duration: 27 minutes