Fig.1   The galaxies in an expanding universe are just like the red dots on the surface of the balloon. When the balloon is inflated, the receding speed between two galaxies is proportional to the distance between them.

The expansion of the universe is now an indisputable fact. Generally speaking, scientists support that, due to gravity, the expansion speed keeps decreasing. But the latest observations have shown that the expansion of the universe is accelerating.

How is the expansion speed of the universe measured? One method is to measure the redshift in the spectra of galaxies [1] so as to obtain their receding speeds from us, and then to measure their distances. In 1929, Edwin P. Hubble discovered that the receding speeds of these galaxies are proportional to their distances by studying the spectral lines of 24 galaxies (Fig.1). This ratio is the well-known Hubble's constant. A larger Hubble's constant represents a faster expansion speed.

However, the universe is not expanding under constant acceleration. Therefore in different periods, the Hubble's constant is different. It is impossible for us to go back to one billion years ago and observe the ratio of the receding speed to the distance of the galaxies at that time. Fortunately, by observing the ratio of the receding speed to the distance of distant galaxies, we can still calculate the expansion speed of the universe in the past. (For example, the light from a galaxy of one billion light years away we see today was emitted one billion years ago, and thus what we see now is actually the outlook of that galaxy one billion years ago.)

We can obtain the receding speeds of the galaxies from the redshift, but how can we obtain their distances? Hubble and his contemporaries assumed that the intrinsic brightness [2] of the galaxies is more or less the same. Those appear to be brighter are closer to us whereas the fainter ones are farther away. But one problem follows. Galaxies of different types and sizes have different intrinsic brightness. In particular, the intrisic brightness of the galaxies that are a few billion light years away (these are the ancient galaxies) has a great difference from that of nearby galaxies, so there is a tremendous difficulty in observing the receding speed to distance ratio of this type of galaxies.

Fig. 2   A schematic diagram showing the expansion rate of the universe is changing. (a) The expansion of the universe is slowing down (b) The exapnsion of the universe is accelerating, i.e., the expansion rate long time ago was smaller than the current expansion rate.

In the last decade, researches in the type-Ia supernova of the scientists shed light to the problem. Type-Ia supernova is formed as follows. When two stars are orbiting around each other, the matter from one of the stars (the partner star) is being slowly attracted towards the other star which is going to explode. When the accreted mass reachees a critical value, this star will have a supernova explosion. All type-Ia supernovae reach about the same absolute maximum luminosity in explosion. Since farther objects look fainter, through observing the apparent brightness of this kind of stars, scientists can estimate the distances of the galaxies from us. Two separate teams of researchers from Australia and America have used this property of supernova to calculate the ratio of the receding speed to the distance of distant galaxies. The ratio of the receding speed to the distance is found to be smaller. In other words, the expansion speed of the universe in the past is smaller than that at present.

Researchers have attempted to use different theories to explain this phenomenon. If no satisfactory explanation can be given eventually, scientists may may have to admit that the universe is really expanding with acceleration. If this is the fact, then what is the power that makes it expand with acceleration? Is there a need to reintroduce the long abandoned cosmological constant [3]?

This question is still requires further discussion, and whether the universe is under accelerating expansion or not would remain a question until more advanced technology is invented so that scientists can take more accurate measurements and thus carry out verifications through observing more distant galaxies.