The Cosmic Microwave Background Radiation is the earliest moment in the Universe that we can see with our telescopes, just a few hundred thousand years after the Big Bang itself. What will it take for us to be able to fill in the missing gap? To see closer to the beginning of time itself?
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- Inflationary epoch (Wikipedia)
- This Timeline Shows The Entire History of The Universe, And Where It’s Headed (ScienceAlert)
Cosmic microwave background (Wikipedia)
- Grand Unification (whillyard.com)
- The Dark Ages and large-scale structure emergence (Wikipedia)
- What are Gravitational Waves? (LIGO)
- BICEP (Background Imaging of Cosmic Extragalactic Polarization) (Wikipedia)
- Laser Interferometer Space Antenna (NASA)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast Episode 551: Missing Epochs: Observing Before the Cosmic Microwave Background Radiation. 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 Dr. Pamela Gay, a Senior Scientist for the Planetary Science Institute, and the director of CosmoQuest. Pamela, how are you doing?
Pamela: I am doing well. How are you doing?
Fraser: I’m doing great! We are recording at funny times. Time and space have no meaning. You are somewhere probably in an airplane or visiting friends somewhere. I might also be in an airplane.
Pamela: It’s a good plan.
Fraser: Attending a Christmas party in California maybe. So, everything’s gonna be up in the air. But we are recording ahead of time so that the schedule when these shows come out have been right on time. That’s the plan. We’re so organized. Yeah, I’ve really got to give all credit to Susie for making sure – someone is thinking far ahead and saying these are the weeks that episodes have to come out. What will it take for Fraser and Pamela to record in advance? Our old plan was to just not or catch up. Susie is ahead of the game.
Pamela: Susie is what keeps us going and all of you who pay into our Patreon program, you are awesome! And you let us pay Susie which really means you let her kids go to college, so double yay.
Fraser: You’re sending Susie’s kids to college. Thank you, from them and us.
The cosmic microwave background radiation is the earliest moment in the universe that we can see with our telescopes, just a few hundred thousand years after the Big Bang itself. What’ll it take for us to be able to fill in the missing gap to see closer to the beginning of time itself? All right, first before we get into what it’s gonna take, what is it?
Pamela: All right, so, last week we talked about how there was the point in time when the cosmic microwave background was formed. Well, we’re now turning the clock back even further. So, when our universe became a thing, went through all these crazy steps of forces dividing and stuff like that, leading into the epic of inflation when there was runaway growth as some of a scalar field propagated through the universe, leading to everything bloating up very rapidly and in a way that causes everything to look basically the same in all directions.
Then after this we were left with a still way too hot, still way too dense universe in which initially you couldn’t even have protons and electrons. Then it cooled enough that you could get those, at which point there was runaway nuclear reactions. We got hydrogen and helium, bits of lithium and beryllium. Then it cooled off enough that all of those nuclear reactions stopped, but you still have this big, dense soup of a universe that has sound waves left over from those nuclear processes propagating through this early universe. And as they do they’re building up areas of higher and lower density, creating what we now see is the power spectrum in the universe.
That’s way oversimplified. Any of you who have a PhD in cosmology are now cringing. We’re doing the big brush strokes here, folks.
Fraser: Okay, so, when you say power spectrum, what does that mean?
Pamela: So, when we look at the cosmic microwave background we see a distribution of places that are a little bit colder and places that are a little bit hotter, and they come in a variety of sizes. And when we look at the distribution of their sizes we see this set of peaks and troughs that we refer to as the power spectrum of the cosmic microwave background. Those sizes are reflective of the characteristics of the early universe.
If we saw a different distribution than what we see it would mean that all of our ideas of how all of this happened are wrong. And luckily when we got our first good measurement using WMAP what we saw matched what we thought we would see, which is always a good thing. And as we’ve continued to revise we have more or less continued to see what we expected. There’s been a few missteps along the way and unfortunately those missteps occasionally get more attention than they should, and are related to the gravitational waves that are the primary topic of today’s show.
Fraser: And when we imagine the cosmic microwave background, the pictures that we see of the cosmic microwave background are this sphere, or sometimes it’s sort of a flattened sphere that has different colors on it, greens and blues and reds and yellows, and when we’re looking at that picture, what are the greens and reds and yellows and blues that we’re all familiar with?
Pamela: Those are tiny, tiny variations in the precise color of light that was released during the creation of the cosmic microwave background, and those slight point-to-point variations in color are characteristic to differences in temperature which map to differences in density at different points in the early universe.
Fraser: So, when we see that ball of universe we’re seeing really just places that were less dense and places that were more dense before the cosmic microwave background. And this is gonna start to help us figure out how we can maybe even begin to probe into this area. And I think when we see that, when we look at that picture we imagine this ball that is growing. And that’s unhelpful, because that’s not what it is, right?
Pamela: Right, no. So, a better way to think of it is if you’ve ever gone walking in the fog with a flashlight, if you hold the flashlight straight up it will illuminate a column of light. Not a sphere, a column. It’s not a perfect analogy. That column that surrounds you which is as far as the light can go before it reflects back off of the fog, that is your visible sphere. And as you move through the fog you take your visible sphere with you. Well, in the case of the cosmic microwave background, as we move through the universe, if we were able to suddenly teleport five billion light-years to the left – whatever the heck light-year – whatever left means in the concept of our universe.
Were we to make this sudden jump, if we observe the universe around us, we would see a different specific pattern of cooler and hotter, longer and shorter wavelengths of light. But the pattern of how many there are of this size, how many there are of this size, how many there are of this temperature, how many of this temperature, that pattern should reproducible no matter where we’re observing it from.
So, we just happen to be seeing the light that was given off by what is now at the appropriate distance. That wall is seeing the light that was given off from where we are right now. This light is traveling in all directions. You’re gonna see it no matter where you go. You’re just seeing a different cutout of the universe.
Fraser: And that idea that when you look off in all directions and you see the cosmic microwave background is all directions, you are just seeing this moment 300,000–350,000 years after the Big Bang, and from everywhere in all directions. And it just happens to be that it’s coming from a certain distance away from you. And then every year that goes by that sphere around you that you’re able to see this is bigger and you’re now seeing more, and the stuff that you had seen last year now you’re seeing the universe that is a little bit older now, and so you don’t see the cosmic microwave background from that spot, because now it’s gone past you.
But now you’re seeing the stuff that’s a little bit older and a little bit farther. It’s such a headscratcher of an idea but until you can get away from that idea of you’re just imagining this sphere that’s expanding and that that’s what you’re seeing, your instincts will always be taking you down the wrong path, because it is not how you’re imaging it.
Pamela: No, and one of the best possible ways to imagine is you are in a ginormous sponge. You carve a sphere out of the sponge and stick yourself in the center and you’re seeing one wall. You pick yourself up, move yourself to a different part of the universe that isn’t within the visible universe you have now, carve out a new sphere, and you’re gonna see a new part of the sponge. But the sponge overall should, if it’s well-made, be basically the same everywhere. Sure, the specifics of where the gaps and density is – the specifics will vary. But the distribution of voids and structure should be reproducible.
Fraser: The reason we can’t see behind the cosmic microwave background is because before that moment everything was opaque. And so, there is this missing 330,000-ish years from the Big Bang to the release of this radiation. We can’t see it. So, how can we see it? How will we find that – how will we learn anything about what happened during that time?
Pamela: Well, we’ve been trying to get everything we possibly can out of the cosmic microwave background light. And one of the hopes was that we would see in its polarization the signature of gravitational waves that would’ve been ricocheting through the early universe that should still be rumbling across the modern universe. Now unfortunately the Background Imaging of Cosmic Extragalactic Polarization instrument, or BICEP, they published all of their results a number of years ago and they were like, we found the polarization. And then everyone looked at the results and said, cool dude, you forgot about dust.
Pamela: They forgot about dust. Well, they didn’t forget about it, they just didn’t realize that what they were actually seeing was polarization due to dust instead of due to gravitational waves. And so, we continue hoping that we’ll get better measurements that are able to overcome the dust, able to instead see polarization due to gravitational waves. But seeing the effect of a gravitational wave isn’t as cool as seeing the gravitational waves itself. And I went on a deep dive down the rabbit hole of papers on what are called primordial gravitational waves in preparation for this episode. And first of all I’m gonna say I found some of the worst written paragraphs and most wordy sentences I have ever come across.
But in addition to having some just really unnecessarily complicated verbiage, what I also saw was the hopes and dreams of a lot theorists who believe that while we’ll never be able to detect these kinds of gravitational waves here on the planet Earth, now that we’ve proven that we can find things on the planet Earth there is a chance that we’ll be able to someday build new and better and able to resolve different frequency of gravitational wave detectors that built in space instead.
Fraser: Right. The follow-on to – LISA might just barely be able to see it, but the follow-on version called the Big Bang Observer, it’s gonna be a mega LISA. And so, that might do the trick.
Pamela: Yes. And LISA itself is – and the plans on this update regularly, so I’m going to be vague for those of you who are listening in the future – LISA is a future telescope, but it’s a future telescope that has been aggressively advertising itself for about 10 years now. Longer than that I think.
Fraser: And one component has already been tested in space, the LISA Pathfinder. So, it’s like half a telescope.
Pamela: It’s making progress sort of for small values of progress. So, LISA is set to be a constellation of satellites that are interconnected through lasers and are shining light between each other and potentially a central detector. And due to very careful understanding of their orbital paths they will be able to differentiate between motions that are due to being in space and orbiting, and motions between the different spacecraft that are instead due to the ripples in the space-time continuum that occur when a gravitational wave passes through.
We actually, as uncomfortable as it is to think about, get stretched and contracted when we are under the influence of gravitational wave. Not at a level that you will ever notice, and since your pants are getting stretched with you unlike on Thanksgiving, it’s not an unpleasant experience. But it’s the kind of thing that we can detect by the changes in how light interferes.
When you shine a light, a laser in particular where you have columniation, depending on how different reflections of that laser interact they will either create a bright spot or a dark spot, and there’s actually a fringe of all of these different bright and dark places in the interference. And as the spacing changes between the different reflections that pattern of interference changes. And that’s what we’re looking for is the change in the interference pattern.
Fraser: And you can kind of imagine LISA is like three buoys floating on the ocean with waves passing over them, and a wave passes over one and then the wave passes over another one, and the distance between the buoys will change as the waves pass over them. And it’s the same thing that’s happening with the interferometers that are happening here like LIGO here on Earth, but just out in space. And the arms will be a lot longer, they’ll be like on the order of 10,000 kilometers. They’re gonna be very carefully tuned. And in theory LISA will be able to see the loudest gravitational waves given off by earlier moments in the universe beyond the cosmic microwave background.
Pamela: And the catch is what is the frequency of the gravitational waves. And this is a really hard thing to imagine. Different objects send a wave of a different size through the universe depending on how they merge or how they come into existence. So, our galaxy on a regular basis is just sort of rumbling out a steady background of really, really low amplitude, we’re never gonna detect them, gravitational waves. Well, during the earliest parts of our universe’s first few hundred years that epoch of expansion, the inflation part, that would have a gravitational wave signal. The sound waves reverberating through the early universe would’ve had a gravitational wave signal.
Now the question becomes how much of this is simply a noise where we can’t make sense of all of these different waves, and what of this can be detected? And then there’s other really cool, intriguing things to think about. We talk periodically about primordial black holes, and if Hawking was correct and black holes are decaying through Hawking’s radiation. None of these things still exist. But we might be able to detect the mergers of early primordial black holes through gravitational waves. We just need to have instruments we don’t currently have.
Fraser: Right. So, that was two separate things. One is maybe there were these black holes that were formed during the Big Bang, shortly after the Big Bang, with over-densities in the early universe. Those might’ve formed those first primordial black holes. And then the ones that we see later on, the ones that form from star collapse or infalling gas, those come later. And so, we could detect those smashing together before the universe was even visible with some of these future instruments, which is a mind-bending idea.
There was one idea for a large-scale gravitational wave telescope that I really liked where one arm – like imagine a big equilateral triangle. One arm of the triangle is at the L4 point – of the Earth-Sun L4 point, one is at the L5 point, and one is at the L3 point, so on the far side of the sun. Which is generally a useless place to put a spacecraft. But you then get an equilateral triangle.
And even though things are moving around and there’s orbits and the Earth’s path isn’t entirely circular around that, by moving the spacecraft in lockstep with each other they should be able to keep them within a few hundred kilometers at the worst point of keeping the legs of this gigantic interferometer together. And so, you make telescope interferometer the size of the Earth’s orbit, a LIGO – a LISA where the legs are more than a hundred million kilometers, which would be great!
Pamela: And this is pretty much the only way we’re going to get any information out about the age of inflation. We need to look at gravitational waves. And gravitational waves also – maybe the only way for to finally put the nail in the coffin of some of the more exotic particle theories. Some forms of string theory, for instance, have gravitational waves from primordial sources occurring in the early ages of the universe. And by looking at what we do see and what we don’t see, that will put restrictions on what the early universe was. And currently we don’t have anything other than that CMB. That’s all we’ve got.
Fraser: Yeah. Now, but the CMB itself, those different temperatures that we see do correspond to over-densities and under-densities in whatever it was before that. So, there are methods that you can use just the cosmic microwave background to probe what must have come before. And then you match that with the math that we have to get a sense of what was going on there, right?
Pamela: And this is where you have to remember that there’s often more than one way to get the same outcome. And theorists will always find wiggle room to come up with a new theory if you don’t give them enough constraints. So, yes, we can do a whole lot looking at the cosmic microwave background to say this is how things should’ve happened. But inflation is still very much a large black box labeled “here be inflation,” with whole lots of different theories to explain it. And it would be nice to have just that one extra point that you have to match.
Gravitational waves can provide hopefully more than just that one extra point. We’re talking about 400,000 years where we went from nothing to everything, and currently we only see the echo of that time. Let’s get in and see that time directly through gravity.
Fraser: So, apart from gravitational waves, are there any other techniques at our disposal that would allow us to see before the cosmic microwave background? It’s only gonna be gravity.
Pamela: It’s only gonna be gravity, because other than gravity we don’t have any forces that carry long distances. So, of the four major forces only gravity and the electromagnetic force carry for great distances. Only light, which is a particle, travels for great distances. We don’t have cosmic rays coming from primordial times. We have the light that was released with the cosmic microwave background, that’s it. That’s all we’ve got.
Fraser: But you’re saying a thing there which is making me think you would say we don’t have cosmic rays. But would there be any particles that could be released? Because particles are being created, protons and heavier elements were being created in that time before the cosmic microwave background, so could there be particles that were being emitted – or not emitted but are still around from that time? Or is that just what – I mean, we know that the universe, by measuring the amount of hydrogen, and the amount of helium, and the amount lithium, et cetera, and beryllium, that tells us what happened during that period. And it perfectly matches the predictions of the Big Bang.
Pamela: There are string theory models. And we all know how I feel about string theory. There are string theory models that say that there could be super strings or other exotic particles left behind that may be detectable. So far they haven’t been.
Fraser: Right, cosmic strings.
Pamela: So, I don’t think so.
Fraser: Right, so, some theories say that there might be structures that are out there in the universe that could explain this.
Pamela: As we do at the end of every episode, I would like to take this moment to thank all of you that make everything we do possible. We couldn’t be here without you and your generous contributions that let us pay Susie, who is our audio producer, our chief cat herder, the person that keeps Fraser and I organized and on track week after week.
And these are the people who’ve been contributing at the acknowledged on YouTube and acknowledged in the show level on Patreon. I would like to thank A Blip in the Universe, Kjartan Sævre, Ed, Steven Shewalter, Gordon Dewis, Bill Hamilton, Frank Tippin, George Thorwald, Richard Rivera, Alexis, Thomas Sepstrup, Fredrik Hogne, Kvam Jensen, Silvan Wespi, Jeff Collins, John Drake, Arctic Fox, Marek Vydareny, John Vance, Nate Detwiler, James Platt, Ron Thorrsen, Phillip Walker, Elad Avron, and Cooper.
Fraser: Thank you everybody, and thank you Pamela. And we’ll see you next week.
Pamela: Sounds great.
Female Speaker: Thank you for listening to Astronomy Cast, a non-profit resource provided by The Planetary Science Institute, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astronomy Cast. You can email us at firstname.lastname@example.org, tweet us @AstronomyCast, like us on Facebook, and watch us on YouTube. We record our show live on YouTube every Friday at 3 PM Eastern, 12 PM Pacific, or 19:00 UTC. Our intro music was provided by David Joseph Wesley, the outro music is by Travis Sural, and the show was edited by Susie Murph.