WE NOW have a clear view of the cosmos. This week, the first results from the Microwave Anisotropy Probe were announced, giving us a clearer picture than ever before of what the Universe is like.
From MAP’s chart of the early Universe, Charles Bennett at NASA’s Goddard Space Flight Center in Maryland and his team have extracted the essential properties of the cosmos. Instead of fuzzy estimates we now know accurately the Universe’s age, rate of expansion and the proportions of its main ingredients. “We now have a standard model of cosmology,” says David Spergel, of Princeton University. MAP has also provided strong evidence that the big bang got its kick from a process called inflation.
Since its launch in June 2001, MAP has been measuring what’s known as the cosmic microwave background. This faint radiation comes from all directions and dates from when the Universe was just 380,000 years old, when the matter that filled space had cooled enough to become transparent. The afterglow has been cooling ever since due to the Universe’s expansion, and its temperature now averages a chilly 2.735 kelvin.
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Measuring its subtle variations gives us a picture of the infant Universe, frozen in time. MAP’s predecessor COBE made headlines worldwide when it first detected the faint ripples in the microwave background in 1992. MAP has now built up a much more detailed picture of that radiation, charting fluctuations in temperature as small as a few millionths of a degree (see above).
The regions of hotter and cooler radiation on this image reflect variations in the density of primordial matter. The denser patches are the seeds of galaxies and other structures in our modern Universe, and hidden in the pattern are the vital statistics of the cosmos. The largest blobs, stretching across several degrees of the sky, are typically about 25 millionths of a degree warmer or cooler than the average. But as you zoom in to smaller scales, the pattern gets more interesting. The strength of the variations changes with the scale, peaking and falling several times as you zoom in.
These “acoustic peaks” hold the most important cosmological clues, as the size and strength of the peaks depend on the physical characteristics of the Universe. For example, dark matter and ordinary matter have different mechanical properties, so the cosmic ratio of the two affects how easily primordial blobs of a given size form or collapse, changing the patterns in the radiation. The position of the peaks also reveals the overall geometry of space, because if space were curved it would act like a lens, magnifying or reducing the apparent size of particular blobs. The blobs don’t seem to be magnified or reduced, so space must be flat.
Different cosmological parameters have overlapping effects on the background radiation, but the team has teased them apart by modelling every imaginable combination. “We generate tens of thousands of different cosmological models – our book of suspects – and compare them with what we see in the data,” says Bennett. And at last, after decades of uncertainty, we have some precise numbers.
The Universe according to MAP is 13.7 billion years old – almost exactly three times the age of the Earth. And we now know its expansion rate: each megaparsec-long stretch of space is getting longer at 71 kilometres per second (a parsec is 3.26 light years). Just four per cent of the Universe is ordinary matter, the stuff of stars and planets, while 23 per cent is dark matter, particles that don’t emit any visible radiation. And 73 per cent is dark energy, which appears to be accelerating the expansion of space.
Perhaps MAP’s deepest insight is into the first split second of the Universe. A rapid period of expansion called inflation was postulated about 20 years ago to explain several known or suspected features of the Universe, such as why space is flat and why the distribution of matter on the largest scales is nearly but not quite uniform. The idea is that a tiny fraction of a second after the big bang, some kind of repulsive force field inflated space by a huge factor, so the entire region of the Universe we see today started as just a sub-microscopic piece of the big bang. The fluctuations in the microwave sky that led to today’s galaxies would have been caused by quantum fluctuations in this original force field.
MAP has now ruled out all other existing theories, because as well as measuring temperature, the probe records the polarisation of the microwaves it receives. While alternative theories predict the strongest polarisation where the temperature fluctuation is strongest, MAP has found the opposite, as inflation predicts.
Now that the old uncertainties of cosmology have been replaced by hard numbers, new questions will take their place. Instead of asking if inflation happened, for example, cosmologists will start to ask why. The theory of inflation currently has dozens of versions suggesting different force fields that would have accelerated the expansion of space.
MAP (recently renamed WMAP after the late David Wilkinson) is beginning to weed out these alternatives. Most of the simplest versions, with only one kind of force field, have been ruled out. Instead, the data favours “hybrid inflation” models, in which one force field gets inflation going and a different one stops it. It is still too early to say what the final answer is, but it’s clear that the early Universe is even more complicated than we thought.
Our Universe: The facts
- • Age: 13.7 billion years
- • Shape: flat
- • Age when first light appeared: 200 million years
• Contents:
4% ordinary matter
23% dark matter. Nature unknown
73% dark energy. Nature unknown - • Hubble constant (expansion rate): 71 km/sec/megaparsec
- To keep the embargo on this data, we were unable to discuss the results with scientists outside the MAP team