star watch

Looking through starry twilight back to Big Bang

As twilight fades in the west through deep dark blue, you see through an enormous range of distances.

The afterglow of day that meets your eye is the bluest component of faraway sunlight being scattered by high-altitude air as much as a few hundred miles in front of you. That’s a distance of about a thousandth of a light-second.

The brightest “star” shining there in the west these evenings is the planet Jupiter, at a distance of 42 light-minutes.


Look lower left of Jupiter to spot Aldebaran, an orange giant star. It’s nearly a million times farther away, 65 light-years. Above Jupiter is El Nath at 130 light-years. To Jupiter’s lower right is the delicate little Pleiades star cluster, 400 light-years from Earth. In a dark sky, you can see fainter stars; a few of these are thousands of light-years out.

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That’s as far as you can go with the naked eye on April evenings. A telescope and a good star atlas can get you out of the Milky Way into the realm of the galaxies, tens of millions of light-years away. The largest observatory telescopes make quick work of galaxy swarms a billion light-years distant and can see back into the first cosmic era of galaxy formation, 12 or 13 billion years back in time.

And beyond that?

The farthest observational wall for astronomy is something that covers the whole sky like a coat of paint. It’s the cosmic microwave background radiation — our view of the white-hot, opaque stuff that filled the universe until just 380,000 years after the Big Bang, when it cooled enough to become transparent. Its light has been traveling free ever since that time, and we see this light in every direction, reshifted by a factor of 1,100 into the microwave part of the spectrum.

The biggest astronomy news this year, to my mind, was the newly detailed map of this sky-covering paint job that the European Space Agency released from its Planck mission on March 12.


Why? Although the microwave background is very smooth — it varies from place to place by only parts per million in brightness and color — it’s not perfect. Look carefully enough and it’s covered with slight spots and ripples of all sizes. Those weak patterns encode rich information about all sorts of things right back to the very start of the Big Bang, and possibly before.

Some of the things that the Planck scientists announced in March were just refinements: tightening up earlier findings drawn from less exquisite maps of the microwave background and other evidence. The universe is now officially 13.8 billion years old, not 13.7 billion — a refinement of less than 1 percent — and the remaining uncertainty has shrunk. We also have a better fix on the nature of the universe’s total contents. Ordinary matter, meaning stars, planets, nebulae, dust clouds, rocks, and everything else made of atoms, amounts to 4.9 percent of all that exists, not 4.5 percent. Dark matter not made of atoms (scientists can only guess what it is) constitutes 26 percent of everything, not 23 percent. “Dark energy” that’s causing the expansion of the universe to speed up — an even bigger mystery — contributes 69 percent, not 72 percent.

All of this stuff adds up to exactly the amount required to make space “flat” and presumably infinitely large, according to Einstein’s general theory of relativity, to an uncertainty now of just 1 percent. Astronomers call results like these “precision cosmology.”

Then come the exciting parts. The ripply microwave patterns covering today’s sky are the fantastically inflated signatures of literally microscopic events that took place in the Big Bang’s first 10-35 second. Their characteristic strengths at different sizes exactly match what’s predicted by the “inflation cosmology” theory of the Big Bang’s driving force. This likely tells where the Big Bang came from: a much larger, super-dense, super-hot, super-expanding material that is eternally spinning off fantastic numbers of other, separate Big Bang universes at every instant. We’re stuck inside just one of them.

But that’s not the only possibility. A competing theory posits that the Big Bang resulted from the collapse of a single previous universe through a “big bounce,” perhaps in the form of two spacetimes colliding in a higher-dimensional realm.


Planck’s data aren’t quite good enough yet to rule on which of those two pre-Big Bang scenarios is correct, if either. But it’s almost there. Planck continues to take data, refining its map of the ripples. Mission scientists hope to be able to say which precosmic picture is correct, or more nearly correct, by the next scheduled data release in 2014.

Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge ( His Star Watch column appears the first Saturday of every month.