Sky Gazing

cycle passes by, or we watch a train passing us at a level crossing, the sound has a higher pitch when the source is approaching, and a lower tone when it is receding. The same applies to light.

If the source is approaching us, its light is shifted to the blue; if it is moving away, its light is reddened, or redshifted. All the elements have unique multicolour signatures, or spectra. For example, in the laboratory we can measure the spectrum of hydrogen, a common element in the universe. Then we search for the spectral signature of hydrogen in distant, cosmic objects.

By comparing the two spectra, we can determine how much the cosmic spectrum is red-shifted, which tells us how fast the object is receding.

If we have the distance and the redshift for an object, and noting that distance is linked to time, we know the speed the universe was expanding at that point in the past.

By measuring the redshift of objects at different distances, we can measure how fast the universe is expanding at different points in its history.

We would expect the expansion after the initial Big Bang to be slowing down.

Instead, surprisingly, it is speeding up. We cannot yet explain this.

————— During the evening Mars, the red planet, is conspicuous low in the southeast. Saturn is low the south and Jupiter very low in the southwest. The moon will reach Àrst quarter on the 16th.