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In these times I feel like I need a bit of a preamble. I’m very thankful to all the diarists who have been keeping us up to date on COVID-19, politically and scientifically. You might know that I love to learn about science and discovery and write about how much fun all that can be, and I hope that doing that provides a little break and lets you think about something hopeful for a little while. I sincerely wish that you’re well physically and mentally, and things are as good as they can be for you under the circumstances.
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The theater of the night sky is about to get even more compelling.
Vera Rubin nailed down the existence of dark matter with her observations of the rotational speeds of galaxies, and now an observatory named for her is getting ready to take our view of the cosmos to a scale and a depth we’ve never seen. It’s going to help reveal, among many other things, the nature of the largest yet least-understood entity in the Universe, dark energy. The survey it will conduct is called the Legacy Survey of Space and Time (LSST). Located in Chile, it’s funded almost entirely ($470 million) by the USA’s National Science Foundation. THESE are the kinds of things that make America great.
Vera C. Rubin Observatory, with its primary mirror measuring 27.5 feet in diameter, is getting very close to completion, and it has already begun taking its first images. COVID-19 will slow the last phases of preparation, to be sure, but let’s look ahead to the other side of this! I’ll get to the latest on the Observatory at the end of the diary, but the Universe is big, and dark energy is mysterious, so let’s get to it…..
We like to say that our Universe came into being 13.8 billion years ago, at the time of the Big Bang, but it’s actually still forming right now. The Universe is expanding, not because galaxies are moving away from each other through space, but because more space is being created all the time, everywhere. Things are being pushed apart because space itself is getting bigger and bigger.
Maybe you’ve seen this type of diagram before:
After the Big Bang had done its thing, people expected that gravity from all the galaxies and dark matter out there might start to pull things back together again and at least slow down the Universe’s expansion, and that probably did indeed happen for awhile. But we found out in the late 1990s that about 7 billion years ago the expansion actually started to speed up, caused by something that hadn't been described before, now known as “dark energy”. Once the matter in the Universe got sparse enough, the dark energy, which was always there, apparently started to take over as the dominant force. Saul Perlmutter, Brian Schmidt, and Adam Riess won the 2011 Nobel Prize in Physics for letting the world know this.
Very weird. An “unexpected plot twist,” said Perlmutter. But how did they figure this out?
They did it by watching 42 supernovae explode! Type Ia (“one-ay”) supernovae, that is.
What they needed to do was find a number of faraway objects for which they could accurately measure two things: 1) their distance from us, and 2) the speed at which they are moving away from us. It’s best to span a large distance if you can, so you get as much information as possible.
Let’s deal with the distance part first.
If I know I’m looking at a 100-watt light bulb, I know how far away it is by how bright it appears to me. Type Ia supernovae are special because they explode in a very predictable way. Their peak brightness is related precisely to the time it takes them to fade out. If you can track the supernova’s explosion over a few weeks, you’ll know exactly how bright it got at its peak. Let’s line up some data from nearby Type Ia supernova explosions that adjusts for the fade period, collected by Mario Hamuy and coworkers, so you can see just how very similar their explosions are:
The Hamuy supernovae were nearby enough that their distances from us were known by other measurements, so their absolute brightnesses could easily be calculated. As if they were 100-watt light bulbs. So now, if you are lucky enough to spot a Type Ia supernova going off, you can measure its brightness to find exactly how far away it was when it blew. That’s why the Type Ia supernova is known as a “standard candle”.
Why are Type Ia supernova explosions all so similar to each other? As shown in the 90ish-second (silent) video below, if you have a pair of stars orbiting each other and one turns into a red giant, it blows off a lot of its material and gets whittled down to a very dense white dwarf. It’s happy existing that way unless it gets heavier than about 1.4 Suns. If its companion star eventually becomes a red giant too, it’ll start donating material to the white dwarf, causing it to slowly get more massive until it reaches that critical limit (called the Chandrasekhar limit). As soon as it does, it collapses and sets off a nuclear fusion chain reaction and kaboom! A nice, predictable process.
But how can we tell that a supernova is a Type Ia when we see it blow up? After all, the other types aren’t this predictable, and we don’t want to make a mistake. Well, its absorption spectrum will have a big honking silicon peak in it that the other types lack (among other distinctions):
So it’s not enough to take photographs; you have to be able to measure all the wavelengths of light. And for faraway supernovae, of course, the signal you get is weak. Not to mention needing to scan large areas of sky to be lucky enough to spot one in the first place. Hence the need for sophisticated surveys like the one that will happen at Rubin Observatory.
OK, now we’ve got some Type Ia’s whose distances we can figure out. Now we need to know how fast they have been moving away from us, and for that we use their redshift. This is where the expansion of space comes in.
In an expanding Universe, everything is moving away from everything else (except for objects that are close enough to each other that gravity pulls them together). The farther away from us something is, the faster it’s moving away from us. If it’s twice as far away, it’s moving away twice as fast. This is easy to visualize if we go to our PowerPoint mini-Universe:
Our square mini-Universe has an edge that keeps growing by 2.4 inches every second. We can draw lines in it, hit ‘Group’ to make it all one object, stretch it out to whatever bigger size we want, then ‘Ungroup’ and measure the lines again. And we see that they all doubled in length, just like the edge of the whole square did, no matter where they were or how long they were. So the farther apart things are, the faster they move apart.
If we divide the speed they’re moving apart by their distance apart, we get the “Hubble constant”, or H. It has the same value everywhere in space, but it can change over time. On the left, you can see that the 2-inch line is going to grow by 2 inches over the next second, so
H = (2 inches/second) / (2 inches) = 1 / second.
In the next frame, when our mini-Universe is 2 seconds old, the 4-inch line is going to grow by 2 inches over the next second again, but now it’s longer, so
H = (2 inches/second) / (4 inches) = 0.5 / second.
You might notice that in a Universe that keeps expanding at the same rate like this, we can divide 1 by the Hubble constant and get a time, and that time will be the age of the Universe.
The REAL Universe appears to have a Hubble constant right now of between 67 and 74. But that’s in kilometers per second per megaparsec. A megaparsec is 3.26 million light-years, so the real Universe is expanding much more slowly than PowerPoint Universe. If we use H = 72 and then straighten out the units, we find that 1 / H = about 13.6 billion years. The Universe hasn’t expanded linearly the whole time, but hey, that’s not too far off.
That value of the Hubble constant means that my cat, who is perched on his kitty condo 10 feet away from me, will take about 4.5 million years to move away from me by about a millimeter! But he won’t even do that, because Earth’s gravity will easily keep us in line.
But if you go far enough away, things will be receding from us faster than the speed of light, not because they are traveling that fast in a conventional sense (can’t do that), but just because the space between us is expanding that much. SO — there are things beyond a certain distance from us whose light will NEVER reach us. They are outside the “observable Universe”. We can never reach them, or even detect them, in any way. There’s something a little sad about that.
Anyway, now we get to the observed redshift of a particular supernova:
We start out in the figure with Earth and a star, 2 length units apart. One unit of time later, space has expanded a bit, so the star is 2.5 length units away from us, and right then it happens to go supernova. That’s when it sends out its bright light, which heads for Earth. I’m just showing the blue part of that light here.
Space keeps on expanding, so by the time that once-blue light reaches Earth, it’s been stretched out, like everything else, and it actually appears red. In this case, every distance throughout space grew by 40% in the time it took the supernova’s bright light to reach Earth, and so the light’s wavelength got longer by that much, too. The redshift z measures the amount of stretch that happened since the light left the supernova. Everything stretched by 40% during that time, so we have z = 0.4.
We know very well what the spectrum of a Type Ia supernova looks like, so to get its redshift, we just measure how shifted over towards red everything in the spectrum is. That big characteristic silicon peak, for example, instead of its normal orange, might appear red or even infrared.
Okay, finally! Now we have brightness and redshift for a whole bunch of Type Ia supernovae! So, to know whether the expansion of the Universe is speeding up, what are we looking for?
Well, if the supernovae that are closer to us have unusually big redshifts, then it means that things have recently been going faster. And that is exactly what our Nobel Prize winners found:
Each dot on the graph is a supernova, and they measured 42 of them, out to a redshift of nearly z = 1. That means the farthest supernovae they measured were ones that blew up when the Universe was only half the scale it is now! Pretty impressive. This is a nice graph because it shows you what you’d expect to see if the Universe has been expanding at a constant rate: all the dots would be right on the border between the blue and orange parts.
But all the dots are in the blue part, the part where supernovae that are far away (low relative brightness) have lower redshifts than you’d expect. In other words, back then the Universe was expanding more slowly, and since then, it must have sped up. The likelihood of all the dots falling in the blue part by chance is approximately zilch-o-rama. Could you flip a coin and get heads 42 straight times? If so, you really need to go to Vegas.
Here, by the way, is some actual supernova data from Perlmutter’s group, showing that when one goes off, it’s about as bright as the entire galaxy it’s in! And it’s got to be that bright to be seen from so very far away.
Whew! So there you have it. The Universe’s expansion is accelerating! And it’s due to dark energy, which is pushing everything apart for some reason, and no one knows what that reason is!
There are other ways to measure this acceleration, too, but I only want to mention one other one because it is really cool: BAOs, or “baryonic acoustic oscillations”.
It turns out that there are giant rings throughout the Universe that are all the same size, and we can see them! When you have rings that are all the same size no matter where they are, you can tell how far away they are by how big they look from here. And you can always use redshift to measure how fast they’ve been moving away from us. So instead of a “standard candle” (brightness), we have a “standard ruler” (size).
Now, why the frick are there same-size rings all throughout the Universe?? It sounds very weird, but they are there; I am not making this up.
I haven’t seen a satisfactory explanation of why they are the exact size they are, but we do seem to grasp generally where they came from.
In the early Universe, it was sooo hot…
HOW HOT WAS IT?
It was so hot … that the cows were giving evaporated milk.
Well, actually, it was SO HOT that there weren’t even any atoms. Protons and electrons wanted nothing to do with each other, and they all just flew around freely. Light will bounce off of free electrons, and so light was also trapped in this mess. The pressure from the light hitting the electrons actually created giant acoustic waves with certain resonant frequencies, so we had these giant ripples in the fabric of the Universe.
Then, when things expanded and cooled down enough, around 400,000 years after the Big Bang, atoms finally formed. The electrons got sucked into those, and the light was suddenly free to go. Without light to push it around anymore, the matter froze in these ripple shapes and has stayed that way ever since. Dark matter wasn’t pounded by the light, so it stayed in the middle of these ripples, and eventually it pulled in some regular matter, so the ripples developed dots at their centers.
With the expansion of space, these ripples grew, too, just like they would in PowerPoint Universe:
And those ripples, a little denser in mass than their surroundings, attracted galaxies and so forth, and today they look like this … well, not quite like this — this is way exaggerated:
Their radii have now swelled out to 500 million light-years across! That is stupefyingly large. But we really can see them, even if we have to take huge surveys and pull out some statistical approaches. They lurk within scans like this one, from the now-complete Sloan Digital Sky Survey (SDSS):
I feel like I can make them out in this image, but I’m probably deluding myself. The way you really know they’re there is to measure every single galaxy-to-galaxy distance within these huge surveys and check to see if there are any distances that are overrepresented.
And ... there are! A large collaboration analyzed the SDSS data and found the now-famous Bump, a galaxy-to-galaxy distance that is represented, among everything with the same redshift, much more than it would be by chance:
So that gives us another method to check out the expansion rate of the Universe over time, but we’ve got to have a really massive survey to do it right. Rubin Observatory can handle this, too!
We’ve had very good surveys already, like the SDSS and the Dark Energy Survey (DES), and those were quite successful — in fact, the DES just found 139 new trans-Neptunian objects to keep Pluto and Sedna and all of our other friends company out there in the Kuiper Belt, and also to give us more clues as to where our 9th planet might be hiding.
The surveys completed up to now are indicating more and more strongly that dark energy behaves like a uniform pressure pushing things out. It’s as though empty space isn’t really empty but possesses some kind of inherent energy. We’ve gotten a remarkable amount of information from these surveys.
Vera C. Rubin Observatory, though, starts the next generation. It’s going to have more than ten times the light-gathering capacity of any survey before it, so we’ll be able to fill in Saul Perlmutter’s blue-and-orange graph much deeper down and with much more accuracy. That will distinguish which models dark energy fits into and which it doesn’t, helping tremendously in our quest to find out what dark energy is.
On January 27 of this year, Rubin Observatory took its first raw image:
The LSST team explains:
On the first night of on-sky observations with the spectrograph, the team started by pointing at the Orion Nebula to make sure that there would be stars and structure in the resulting images. [Here] is the raw image, without any processing, as it first appeared on the screen.
The mount assembly for the main telescope, shown as it was about a year ago, outside of its observatory building, so you can see its scale:
And here it is within the new main building, as of a couple weeks ago (March 6):
The entire project is on schedule and actually a bit under budget! It’s collecting usable data already, but it’s going to keep ramping up operations and be fully underway in 2022.
Vera C. Rubin Observatory is an order-of-magnitude jump in our capability to investigate the Universe. Its namesake would be so excited!
It’s going to be a blast!