we know and loveâbut what had spawned the inflaton? Some other mysterious field? And behind that? Was it turtles all the way down? I was about to ask her when a few other physicists approached. âWe found a restaurant. Letâs go get lunch.â
Chatting with Lisa Randall at UC Davis
Dan Falk
They seemed to be speaking to me, tooâor at least they hadnât specifically asked me
not
to come, which I figured was as good as an invitation. So I tagged along and soon found myself at a long table in a casual Italian restaurant with Sir Martin Rees, Britainâs Astronomer Royal; David Spergel, a Princeton physicist who played a key role in analyzing the WMAP results; Randall; and a few legit journalists.
After everyone placed their orders, the conversation turned to the dreaded A-wordâdreaded but looking less avoidable by the day, given its ability to explain the inexplicable.
Like dark energy. Physicists knew from the supernova data, and now from WMAP, that dark energy is extremely sparse, a meager 10 -23 gram in every cubic meter of space, barely a whisper in the dark void of the vacuum, but a whisper that builds with distance and at large enough scales crescendos to an audible howl.
Thatâs because the most likely identity of the dark energy is the energy inherent to the vacuum of space itself, christened by Einstein as the âcosmological constant.â Its power lies in its constancyâas space grows, everything in it gets diluted out,
except
for the dark energy, whose density remains constant. More space, more dark energy: the kind of feedback loop that takes off running.
Youâd think that physicists could have predicted the observed strength of the dark energy, given everything they already know about the quantum vacuum. Quantum field theory provides all the tools you need to calculate the vacuumâs energy. Unfortunately, the calculation comes out wrong.
Really
wrong. According to the theory, the vacuum energy should be infinite. Clearly itâs not infinite, otherwise weâd all have been ripped to shreds by the blazing expansion of space. Since objects arenât spontaneously combusting all around us, the vacuum must be a reasonably calm place, at least at atomic scales and bigger. So if itâs not infinite, physicists had figured, it ought to be zero.
That sounded like a weirdly big leap, but zero and infinity are more similar than youâd think. They are the simplest and most elegant quantities to calculate. Itâs far tidier to come up with a theory that suggests that some number should be zero or infinity as opposed to, I donât know, 3,746. Finite numbers can seem pretty random. So if infinity was off the table, zero seemed like the next best choice. Physicists figured that there could be some feature of the vacuum with positive and negative contributions in equal numbers, canceling out to a perfect zilch.
But that was before astrophysicists traded pencils for telescopes and actually measured the value of the dark energy, finding that it was almost zero, but not quite. It was the worst kind of number: tiny but finite. Getting the right value would require some mechanism that could take quantum field theoryâs infinity, cancel it to zero out to 120 decimal places, and then miraculously stop, leaving some minuscule crumbs behind. Crumbs that could hijack the universe.
A number that fine-tuned is rare, to say the least, and physicists hadnât been able to dredge up a single good explanation. In desperation, they turned to the
A
-word. As it happens, dark energyâs bizarrely fine-tuned value fits squarely in the narrow range that would allowatoms, stars, carbon, and eventually life to exist. A little larger or a little smaller and our Goldilocks existence would be shot. In itself, that observation makes the whole situation worseânow not only is it an incredibly unlikely value, itâs also, coincidentally, exactly the kind of
Chatting with Lisa Randall at UC Davis
Dan Falk
They seemed to be speaking to me, tooâor at least they hadnât specifically asked me
not
to come, which I figured was as good as an invitation. So I tagged along and soon found myself at a long table in a casual Italian restaurant with Sir Martin Rees, Britainâs Astronomer Royal; David Spergel, a Princeton physicist who played a key role in analyzing the WMAP results; Randall; and a few legit journalists.
After everyone placed their orders, the conversation turned to the dreaded A-wordâdreaded but looking less avoidable by the day, given its ability to explain the inexplicable.
Like dark energy. Physicists knew from the supernova data, and now from WMAP, that dark energy is extremely sparse, a meager 10 -23 gram in every cubic meter of space, barely a whisper in the dark void of the vacuum, but a whisper that builds with distance and at large enough scales crescendos to an audible howl.
Thatâs because the most likely identity of the dark energy is the energy inherent to the vacuum of space itself, christened by Einstein as the âcosmological constant.â Its power lies in its constancyâas space grows, everything in it gets diluted out,
except
for the dark energy, whose density remains constant. More space, more dark energy: the kind of feedback loop that takes off running.
Youâd think that physicists could have predicted the observed strength of the dark energy, given everything they already know about the quantum vacuum. Quantum field theory provides all the tools you need to calculate the vacuumâs energy. Unfortunately, the calculation comes out wrong.
Really
wrong. According to the theory, the vacuum energy should be infinite. Clearly itâs not infinite, otherwise weâd all have been ripped to shreds by the blazing expansion of space. Since objects arenât spontaneously combusting all around us, the vacuum must be a reasonably calm place, at least at atomic scales and bigger. So if itâs not infinite, physicists had figured, it ought to be zero.
That sounded like a weirdly big leap, but zero and infinity are more similar than youâd think. They are the simplest and most elegant quantities to calculate. Itâs far tidier to come up with a theory that suggests that some number should be zero or infinity as opposed to, I donât know, 3,746. Finite numbers can seem pretty random. So if infinity was off the table, zero seemed like the next best choice. Physicists figured that there could be some feature of the vacuum with positive and negative contributions in equal numbers, canceling out to a perfect zilch.
But that was before astrophysicists traded pencils for telescopes and actually measured the value of the dark energy, finding that it was almost zero, but not quite. It was the worst kind of number: tiny but finite. Getting the right value would require some mechanism that could take quantum field theoryâs infinity, cancel it to zero out to 120 decimal places, and then miraculously stop, leaving some minuscule crumbs behind. Crumbs that could hijack the universe.
A number that fine-tuned is rare, to say the least, and physicists hadnât been able to dredge up a single good explanation. In desperation, they turned to the
A
-word. As it happens, dark energyâs bizarrely fine-tuned value fits squarely in the narrow range that would allowatoms, stars, carbon, and eventually life to exist. A little larger or a little smaller and our Goldilocks existence would be shot. In itself, that observation makes the whole situation worseânow not only is it an incredibly unlikely value, itâs also, coincidentally, exactly the kind of