Transit of Venus and our expanding universe

Tomorrow Venus will pass between the sun and the earth for the first time since 2004 and for the last time until 2117. Because the rota­tions of the earth and Venus around the sun are so sim­ilar — 365 and 225 days, respec­tively — it takes a while to catch up with Venus in this way, said North­eastern physics pro­fessor Brent Nelson.

The event is sim­ilar to a solar eclipse, but because the planet is so far away it looks like a tiny dot trav­eling across the sun’s surface.

To the naked eye, the sun and the moon appear to be the same size, so a solar eclipse com­pletely blocks the sun. The two have the same “angular size,” which is to say that if you draw a tri­angle between your posi­tion on earth and the posi­tion of the moon in the sky at two dif­ferent points in time, the angle of that tri­angle will be the same as if you drew a sim­ilar tri­angle with the sun. The ancient Greeks used solar eclipses and trigonom­etry to esti­mate the dis­tance between the earth and the sun.

But the angular size of Venus is quite a bit smaller than that of the sun — 1/​30, to be pre­cise. “That’s right at the limit of what the human eye can really resolve,” said Nelson. “If it wasn’t so bright you might actu­ally sense that it has size to it. When you see it in relief against the sun as a dark spot, it becomes quite apparent how big it is.”

The first time anyone observed the transit of Venus, back in 1639, they used the event to revise the ancient Greek cal­cu­la­tion. Appar­ently they deter­mined that the sun was much far­ther away than they had pre­vi­ously thought.

There was a lot of unease about the sense that the solar system was so vast,” said Nelson. “This ever growing dis­tance between us and the other heav­enly bodies was starting to create a sense of dismay. How can we be this tiny rel­a­tive to all the rest of these things?”

One of the most dif­fi­cult and impor­tant things in astronomy and astro­physics, Nelson said, is asking the ques­tions “how far away is that thing?” As you move father and far­ther away, you need to employ dif­ferent tools to answer that same question.

One of those tools is observing the chem­ical com­po­si­tion of an object using its gaseous spec­trum and then mea­suring its fre­quency. If that fre­quency is lower than what it would be on earth, you can assume it is moving away from us.

In the 1920s, physi­cists real­ized that all of the very dis­tant bodies — those out­side of our solar system — were moving away from us (with the excep­tion, appar­ently of the nearest galaxy, Andromeda). They dis­cov­ered that the uni­verse was expanding. In Einstein’s day, the source of that expan­sion was called the cos­mo­log­ical constant.

I asked Nelson if that notion ever over­whelmed him. “I work on string theory,” he said. “So I’m required to imagine ten dimen­sions at a time — which is impos­sible of course. I’m used to the fact that nature need not be easily intel­li­gible.” Ulti­mately, he said, we can figure it out math­e­mat­i­cally, but what that really means in a tan­gible sense is much harder to under­stand. The expan­sion of the uni­verse is one of these weird ideas that is hard for the human brain to imagine. It implies that it’s expanding into some­thing. But that’s not quite right, he said. It only means the dis­tance between any two points in space-​​time is expanding and that’s it.

In 1999 physi­cists revealed once again that the uni­verse is incred­ibly dif­fi­cult to com­pre­hend. They found that the cos­mo­log­ical con­stant, which describes the energy of the empty space (the space cre­ated by the expanding uni­verse) was not zero. “No one wanted to believe it,” said Nelson. “We wanted to believe that the cos­mo­log­ical con­stant was zero because we had pre­cisely zero good ideas about it.”

Even weirder than the fact that it’s not zero was the fact that it’s not huge, either. “It’s right in this sweet spot,” said Nelson, where we’re accel­er­ating but not so fast as to blow the uni­verse to pieces in a frac­tion of a second, in which case “there never would have been chem­istry or life or planets or any­thing at all.”

It begs the ques­tion — what would the cos­mo­log­ical con­stant have to be to create a cat­a­strophe? It turns out that if the cos­mo­log­ical con­stant were only two or three times bigger, I wouldn’t be writing about it, Ein­stein would never have thought of it, and the dinosaurs would never have gone extinct because they would never have existed in the first place. If the cos­mo­log­ical con­stant were dif­ferent, there would be no universe.

Well, that’s a lucky coin­ci­dence,” said Nelson. Some start going toward intel­li­gent design as a solu­tion, but others are trying to explain it math­e­mat­i­cally. One expla­na­tion is string theory, which can be fun­da­men­tally defined as a quantum theory of gravity. “It’s per­fectly designed to address cir­cum­stances where both enor­mously mas­sive bodies and the very very small can coexist,” he said.

That’s what the uni­verse is expected to have looked like in the year of the big bang, before the uni­verse began to expand, when it was still totally com­pacted. “So there may be things in the uni­verse now that bear the imprint of string theory.”

The first obser­va­tion of the transit of Venus revealed that the uni­verse was much larger than pre­vi­ously believed. It was, in some ways, the birth of modern cos­mology, Nelson said.

Tomorrow after­noon, begin­ning at 5pm and lasting until sunset, we’ll be able to watch a sort of astro­nom­ical home video of that his­tory. Think of how much changes in our under­standing of the uni­verse between each transit. How dif­ferent will things look next time? Maybe string theory will be a totally unex­citing idea that everyone takes for granted. Maybe we’ll be able to imagine ten dimen­sions by then, just the way we can imagine a spher­ical earth much better than our fore­bears who believed it was flat.


Now, an impor­tant word of cau­tion, though: DO NOT LOOK DIRECTLY AT THE SUN tomorrow or ever. If you want to watch the transit of Venus, please take pre­cau­tions like wearing solar viewing gog­gles or set­ting up a pin­hole pro­jector and watch its shadow. Or, watch it online in real time with SLOOH Space­Camera.