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COMPUTING SCIENCE

A Box of Universe

Watch the cosmos evolve in a cube one billion light-years wide

Brian Hayes

A History of the History of the Universe

Our view of the universe—and of our own place in it—was famously upset by the Copernican revolution of the 16th century. But the past 100 years have seen even more radical upheavals in cosmology.

It began in the 1920s with the recognition that spiral “nebulae” are not dusty pinwheels scattered among the nearby stars; the nebulae, which we now call galaxies, are made of stars, billions of them, and they lie at immense distances. Furthermore, we too inhabit just such a galaxy.

Edwin P. Hubble, examining the spectra of galaxies, found that almost all of them are shifted toward the red end of the spectrum. The redshifts indicate that the galaxies are moving away from us, and Hubble showed that their velocity is proportional to their distance. The whole universe is expanding.

The expansion could be traced backward to a moment of origin, the Big Bang, which meant the universe must have a definite age (now estimated at 13.7 billion years). The clinching evidence for the Big Bang model was the discovery in 1965 of cosmic background radiation—a relic of the early, incandescent universe, now cooled to a temperature of 2.7 kelvins, with peak emission in the microwave region of the electromagnetic spectrum.

By the 1970s the Big Bang model was firmly established, but nagging problems remained. Looking around us, we see a well-stirred porridge—the same in all directions—and yet the universe is too young to be so thoroughly mixed. There are regions that have never communicated because speed-of-light signals have not yet had time to pass between them. How did these disconnected pieces all come to have the same temperature? Explaining the uniformity called for another dramatic revision in the history of the early universe: a split-second episode of “cosmic inflation” so rapid that a tiny (and therefore nearly uniform) patch of space expanded to become what is now our entire observable universe.

Then there was the mystery of the missing mass. The orbital velocities of stars within a galaxy depend on the total mass of the galaxy and how that mass is distributed. On a larger scale, the motions of galaxies within a cluster also depend on the cluster’s mass. Measurements of both kinds of velocities suggested there is more matter out there than meets the eye. Galaxies must be enveloped in huge halos of “dark matter,” unseen because it neither emits nor absorbs electromagnetic radiation. The nature of this substance remains uncertain, but it apparently constitutes 80 percent of all the matter in the universe.

A little more than 10 years ago came yet another big surprise. Two groups of astronomers had set out to measure changes in the rate of the cosmic expansion over the history of the universe. Gravitational attraction acts to slow the expansion, and might even cause an eventual reversal and collapse. But the measurements showed that the expansion is actually accelerating: Something is pushing space apart. That something has been given the name “dark energy,” and it appears to be the majority constituent of the universe.

All of these ideas are elements of the current consensus view of cosmology called the ΛCDM model. The Greek letter Λ (lambda) is the symbol Einstein chose for his “cosmological constant,” which offers one way of understanding the dark-energy phenomenon. CDM stands for cold dark matter. The matter is cold in the sense that it consists of massive particles moving at modest speeds, rather than a high-temperature gas of nearly massless particles.

In the ΛCDM model, almost 73 percent of the substance of the universe is dark energy, and about 23 percent is dark matter. Thus more than 95 percent of the universe is stuff that wasn’t even known to exist a mere 30 years ago. Our own substance, called “baryonic” matter after the class of particles that includes the proton and the neutron, makes up 4.6 percent of the total. Most of that is hydrogen gas in the intergalactic medium. The stars in galaxies account for 0.4 percent.




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