The early universe ran in slow motion

Time is relative, as they say, especially for mid-day meals. As special relativity shows, the measurement of any two clocks depends on their motion relative to each other. The higher their relative speeds, the slower each clock is relative to each other. So, since we see distant galaxies moving away from us, we should also see time moving more slowly. Right?

While this is true, we should be careful not to confuse what is happening. One of the central ideas to note is that every observer will experience time at the standard rate. Watch the clock as you zoom in on a rocket and the seconds are ticking away just like they always do. The same is true for an observer on Earth. It’s only when an observer on Earth and in a speeding rocket ship compares their clocks that things get confused. Each observer thinks the other has a slow clock.

The other thing to keep in mind is that special relativity doesn’t apply globally when things like gravity and dark energy come into play. For that, you need general relativity, but the results are pretty much the same. In the case of distant galaxies, they are not rocketing away from us through space. Instead, space itself is expanding between us and those galaxies. Due to this cosmic expansion, their light is redshifted, hence their high redshift. This cosmic expansion also lengthens the apparent time between ticks and tocks, making the pace of time appear slower in those galaxies.

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For this reason, one of the predictions of cosmology is that when we look at distant objects, peering into the deep past universe, time should appear to move more slowly. In our view, time was slower in the early universe. Much slower.

As strange as that is, we have evidence to back it up. When we look at distant supernovae, we see a time dilation effect. Type Ia supernovae are used as standard candles because they have a constant maximum brightness, but they also have a coherent light curve, meaning they all brighten and fade over a similar period of time. Astronomers have shown that the further away a Type Ia supernova is, the more its light curve is elongated. In other words, a distant supernova takes longer to light up and fade out than a closer supernova. Time seems to slow down with distance.

According to the standard cosmological model, this correlation between distance and time dilation should go all the way back to the cosmic microwave background. And while all the evidence we’ve gathered supports this theory, our evidence has been somewhat limited. The most distant supernovae we have observed come from a time when the universe is just under 4 billion years old. But a new study has demonstrated the model for even greater distances, thanks to quasars.

Width of supernova light curve versus cosmic distance. Credit: Goldhaber, et al

Quasars are extremely powerful active galactic nuclei (AGNs) powered by supermassive black holes in early galaxies. They are called quasars or quasi-stellar objects because they were first discovered as bright points of radio light. Quasars don’t emit light in a single flash like supernovae, but they do have a kind of time clock. Due to the finite speed of light, it takes some time for effects to traverse the arc of an AGN. For this reason, the intensity fluctuations of a quasar depend on the size of the AGN. So quasars of the same size fluctuate in intensity at the same rate.

Prior to this recent study, there was no way to distinguish whether the slower floating quasars were an effect of time dilation or were simply larger AGNs. Then the team looked at 190 distant quasars at a range of wavelengths. They combined 20 years of data collected at the green, red and infrared wavelengths and were able to create a standard tick-and-tock measure of quasar fluctuations. When they looked at the quasars by distance, there was statistically a correlation. The more distant the quasar, on average the slower its clock.

The study was able to extend the observations of time dilation to when the universe was just a billion years old. From our point of view, one second of that era seems to last five seconds. This agrees with general relativity and the standard cosmological model.

While not unexpected, the result is another wonderful way to show us that we understand cosmic evolution. Space is indeed expanding, fueled by dark energy, and the universe did indeed start with a big bang, where time seemed to move much slower than it does now.

Reference: Goldhaber, Gerson, et al. “Parametrization of time-scale elongation of type Ia supernova B-band light curves.” The Astrophysics Journal 558.1 (2001): 359.

Reference: Lewis, Geraint F. & Brewer, Brendon J. “Detecting Cosmological Time Dilation of High Redshift Quasars.” Nature astronomy (2023).

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