The red circles represent the decreasing rate of expansion of the universe. For 7.5 billion years, the expansion accelerates (green circles). (Source: Ann Feild, STScI)
Read aloud For about four years, the cosmologists assume that the universe expands faster and faster. Then the evidence hardened that this was not always the case: in the first few billion years of the universe, the expansion was slowed down. Michael Turner of the University of Chicago and Adam Riess of the Space Telescope Science Institute (STScI) have now shown that the supernova SN detected by the Hubble Space Telescope in 1997 confirms this view. The researchers present their findings in The Astrophysical Journal (vol. 569, p.18). "This supernova tells us that the universe behaved like a car driver slowing down as he approaches a red traffic light and then presses the accelerator pedal when the traffic light turns green, " Riess commented a year ago, the first evaluations of the Supernova images. Now, together with Turner, he has calculated under what conditions this conclusion is correct.

Their result: The picture that the Supernova SN 1997ff draws from the first decelerating and then accelerating universe is correct with a probability of 90 percent. This result is valid only on the assumption that the mysterious "dark energy" responsible for the acceleration is identical with the energy of the vacuum or even with the cosmological constant, as Albert Einstein originally called this energy.

These conclusions are only possible because all Type Ia supernovae are approximately equally bright. The differences in luminosity amount to a maximum of 12 percent. The astronomers call these supernovae therefore "standard candles". If such a supernova appears darker than another, then you know that it has to be farther away than the brighter one. Due to the incoming brightness we can calculate the distance.

Regardless, one measures the so-called redshift of light. As the universe expands, the light waves are lengthened: all colors shift towards the long-wave red. From the magnitude of this redshift, one can now infer the expansion velocity of the universe. display

In summary, the following picture emerges: The supernova SN 1997ff exploded 10 billion years ago at a time when the speed at which the universe expands still decreased. About 7.5 billion years ago, dark energy gained the upper hand over the braking gravity of matter, and since then, the rate of expansion of the universe has been increasing.

An animation to expand the universe can be found here.

Axel Tilleman

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