How do we know the universe is expanding? Why can鈥檛 there be an alternative explanation, say that all atoms are shrinking?
Richard Swifte, Darmstadt, Germany
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We can鈥檛 be sure that the universe is expanding, but it is the most likely explanation, given the available evidence. The hypothesis is based on the observed 鈥�red shift鈥� of light from distant galaxies towards longer wavelengths.
A star鈥檚 intense radiation causes the electrons of atoms near its surface to change their orbital positions and absorb radiation at discrete frequencies. Each element produces a unique set of spectral absorption lines, like a barcode, which can be compared with standard measurements to calculate the amount of red shift. This is analogous to the Doppler effect of, say, a vehicle鈥檚 sound changing frequency as it moves towards or away from you. The increase in red light suggests that the stars are moving away from us.
An alternative idea was proposed by cosmologist Christof Wetterich in 2013. He suggested that the mass of atomic particles could be continuously increasing, as their size correspondingly shrinks, and that this is also compatible with the observed red shifts. However, Wetterich鈥檚 theory isn鈥檛 taken too seriously, probably because most scientists prefer to adopt the simplest valid explanation that fits observations 鈥� a guiding principle known as Occam鈥檚 razor.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
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