The Universe is Expanding! Is Time Passing Faster Now Than in the Early Universe?

In July 2023, two astronomers from the University of Sydney and the University of Auckland monitored and analyzed quasar light variation signals from the early universe (about 1 billion years after the Big Bang) and found that the duration of these signals when they arrived at the Earth was 5 times longer than when they were produced in the early universe.

Many people may interpret such observation results as "time is passing faster now than in the early universe". Is this really the case? How should we correctly understand this research result?

Expanding Universe

In the early 20th century, astronomers discovered that the distant galaxies around the Earth were moving away from us, and the farther away the galaxies were from the Earth, the faster they were moving away from the Earth. If we assume that the Earth's position in the universe is not special (or that the universe is isotropic), such a picture can also be observed in other galaxies (although we cannot reach there at present). In other words, all galaxies are moving away from each other. This means that the universe we live in is expanding!

The distant universe as seen by the Hubble telescope. Image credit: NASA apod

An expanding universe cannot be imagined intuitively. As an analogy, we can imagine a balloon that is expanding and filled with ants. As the balloon expands, any ant will find that its fellow ants are moving away from it. This result is caused by the expansion of the space in which the ants are located.

Observations of the expanding universe show that our universe has not remained static since ancient times, but is dynamically changing.

Schematic diagram of the expansion of the universe. When observed from any position in the universe, the surrounding galaxies are moving away from you. Image source: science-sparks, annotations modified and added by the author

It should be noted that the effect of cosmic expansion can only be manifested when spanning a sufficiently large spatial scale.

In a bound system in a smaller local area, matter will not expand as space expands. For example, we know that the body of an ant on an expanding balloon, as a self-bound system, will not expand as the balloon expands.

Similarly, celestial bodies in a local area of ​​the universe will not expand with the expansion of the universe. For example, our Earth, the Sun, the Milky Way and other celestial bodies have not expanded with the expansion of the universe. Even for larger-scale galaxy clusters (self-gravitationally bound systems composed of galaxies), they will not expand with the expansion of the universe. In our galaxy cluster, the Milky Way and its neighbor, the Andromeda Galaxy, are not moving away from each other, but are getting closer due to gravitational attraction. Therefore, to explore the expansion effect of the universe, it is necessary to conduct research on a sufficiently large spatial scale.

Quasars: Brighter, More Distant Probes of the Universe

To observe celestial bodies deep in the universe, the celestial bodies need to be bright enough when observed. For distant celestial bodies, their luminosity (energy generated per unit time) must be very high.

There is a type of supernova called Type Ia, which releases huge amounts of energy in a short period of time and has the same maximum luminosity and similar light curves. Astronomers have used this type of supernova to detect the effect of cosmic expansion in the local universe. However, at greater distances, supernovae are difficult to observe, making it difficult to study the conditions in the more distant universe.

Quasars in the early universe. Image credit: NASA

Quasars are a more distant and brighter celestial body. The supermassive black hole at the center of this type of celestial body releases huge amounts of energy by accreting surrounding matter. Many quasars are very far away from the Earth, and it often takes tens of billions of years for the light they emit to reach us.

In other words, the light from these celestial bodies that we observe on Earth was emitted in the early universe billions of years ago. Such distant celestial bodies make it possible for us to trace back to the past of the universe and study its historical evolution.

By observing distant and bright quasars, we can trace back to the early universe billions of years ago. Image source: spaceaustralia, annotations modified and added by the author

From "tick-tick" to "tick-tick-tick"

When the central black hole of a quasar accretes surrounding matter, the luminosity often changes due to the instability of the accretion disk, resulting in bright and dark flickering in observation. Since quasar light variation signals often come from the early universe, astronomers track the expansion history of the universe by recording the duration of these flickering signals on Earth.

In July 2023, two astronomers from the University of Sydney and the University of Auckland studied the light variation (luminosity change over time) signals of 190 quasars. The light variation of each quasar has been observed hundreds of times in multiple bands over the past 20 years.

The research results were published in the journal Nature Astronomy. Image source: Nature Astronomy

Statistical analysis shows that the observed light variation signals of these quasars last five times longer than when they were produced in the early universe (12 billion years ago, or about 1 billion years after the birth of the universe).

In effect, it is like a signal that is delayed from a short "tick-tick" when transmitting to a long "tick-tick-tick" when receiving. If a 1-minute video is recorded on a distant quasar and transmitted to the earth through electromagnetic waves after 12 billion years and received by us, when we play this video now, we see a set of slow motion, and it takes 5 minutes to play this video.

The duration of the signal from the past has been extended by 5 times when we receive it, as if time now is passing 5 times faster than time in the past.

However, this is not actually because the speed of time passing has actually increased, but rather it is an effect caused by the expansion of the universe during the propagation of light signals.

For a light signal, if it propagates in a non-expanding universe, the duration of its transmission and reception is the same; but when the signal propagates in an expanding universe, it takes longer to receive the signal because the propagation distance increases. This effect is called the "cosmic time dilation effect" in astronomy.

This effect will further cause changes in some physical quantities in astronomical observations, such as the observed wavelength becoming longer (called cosmological redshift), the observed brightness of celestial bodies becoming dimmer, and so on.

A signal sent by a quasar in the early universe was transmitted to the earth. Because the universe is constantly expanding, the duration of the signal was extended when it reached the earth. Image source: flat universe society, annotations added by the author

It should be clarified that the cosmic time dilation here is not the same as the time dilation caused by the reference frame transformation in relativity. During the expansion of the universe, celestial bodies are stationary in the cosmic co-moving coordinate system (such as the longitude and latitude lines shown in the figure above, which expand or contract with the sphere).

Although we observe that distant celestial bodies are moving away from us, this is because the celestial bodies are passively "drifting with the flow" as the universe expands. There is no relative motion between these distant celestial bodies, so there is no time dilation caused by relative motion.

In addition, when we talk about the speed of time passing, we are referring to the time of a local stationary clock. The speed of time passing for a stationary clock somewhere in the early universe is the same as that of a stationary clock on Earth now. Therefore, the results of this study mean that "the duration of the signal we receive today is longer than the duration of the signal when it was sent in the early universe", and it should not be misunderstood as "the speed of time passing now is faster than that of the early universe".

Conclusion

Whether it’s billions of light years away or a mysterious ticking sound, from the early universe to the present day, the field of astronomy has countless mysteries waiting to be explored.

How many mysteries are there in the universe? What is the biggest mystery in the universe? No matter what scientists have discovered, the universe always seems to be in a state of "to be discovered". It is this process from unknown to known and then to unknown that is driving the continuous progress of human civilization.

References

[1] Lewis, Geraint F., Brewer, Brendon J.; Detection of the cosmological time dilation of high-redshift quasars; 2023NatAs.147L;

[2] Time Flowed Five Times Slower Shortly after the Big Bang;

Planning and production

Produced by Science Popularization China

Author: Li Zhenzhen, Xiong Yifei, Shanghai Astronomical Observatory, Chinese Academy of Sciences

Producer丨China Science Expo

Editor｜Yang Yaping