Matthew Kleban discusses the arrow of time and the evolution of the universe

Theoretical physicist Matthew Kleban, from NYU

The first conference of the Intercontinental Academia on the subject of “Time”, on April 21, addressed what is known about the history and the possible future of the universe, as well as the concept of “arrow of time,” which posits only one direction for the flow of time, and considers that past and future are different, a notion closely related to cosmology.

The lecturer was theoretical physicist Matthew Kleban, from NYU, who dedicates himself to the study of string theory and the early history of the universe.

He noted that, contrary to what is assumed by the arrow of time, there are physical laws that posit dual direction, i.e., symmetry between past and future, although he stressed that this idea is still very confusing to physicists.

Entropy

According to Kleban, if this idea is correct, then the difference between past and future must be related “to an ‘environmental’ aspect, to an accident of history, such as the difference between North and South Americas, albeit universal, applicable anywhere and anytime.”

When entropy (“disorder”) is low, it tends to increase and the direction of its increase defines the future, said Kleban. Thus, entropy, which was very slight in the early universe, is the “environmental” factor that distinguishes the past from the future. “However, nothing prevents the arrow of time from having a reverse movement due to some other ‘environmental’ aspect.”

Cosmology

With regard to cosmology, he first defined what this science is: “The branch of astrophysics that studies the structure of the universe in the largest accessible scale; this includes the study of the birth, death (or future) and evolution of the Universe over time.”

However, because the universe is 14 billion years, we can only see a portion of it, although quite vast, corresponding to the distance traveled by light in these 14 billion years, he said. “Looking at the past, we see that the universe was hotter and opaque 14 billion years ago, so we cannot see (at least directly) its birth.”

Continuing with his presentation, Kleban addressed the current contents of the universe. He said there are about 100 billion galaxies, each with hundreds of billions of stars. The Milky Way has nearly 300 billion stars and a colossal black hole exists in the center of the galaxy, called Sagittarius A*, with mass equivalent to 4 million Suns, but with radius (at least in theory) only 17 times the size of the Sun. The solar system also orbits the galactic center, but the orbit lasts 200 million years.

Hubble's Law

The panel on galaxies was used by Kleban to introduce his comments about the expansion of the universe. The central figure here was American astronomer Edwin Hubble (1989-1953), whose telescope in the 1920s was able to observe approximately 50 galaxies (the Hubble Telescope, orbiting the Earth for 25 years, allows us to observe 10,000 galaxies when aimed at each 10/1,000,000 slit of the heavens).

Kleban explained that Edwin Hubble noticed something odd in the galaxies: the farther away they were, the faster they moved away from Earth. This observation led to the so-called Hubble’s Law: v = Hd, where H is a constant with units of 1/time. With this law, it became possible to calculate when the entire content of the universe was bundled together, so to speak: 14 billion years ago.

Kleban then reversed the arrow of time, as if the history of the universe moved backward, from when galaxies were just gas, through the increased warming, the opacity, the nucleosynthesis of protons and neutrons of helium and lithium, the inflation (when the volume of the universe spiked dramatically in a tiny fraction of a second), until reaching what is known as the singularity, “where even mere speculation collapses.”

He pointed out that each of these phases of the universe produced enormous entropy and that, even today, entropy is increasing. “Life can be seen as a process that accelerates the production of entropy, as stars and black holes do even more so.”

Hubble’s observations about the expansion of the universe created a profoundly strange idea, namely, the notion of the Earth as the center from which everything moves away, a kind of resumption of Ptolemy’s geocentrism.

Relativity

Kleban explained that in 1916, ten years before Hubble’s observations, Albert Einstein (1879-1955) developed the General Theory of Relativity, a sequence to the Special Theory of Relativity (1905), which had unified space and time (and energy and momentum). “In Einstein’s Theory of Relativity, time is relative, elapsing slower for an object moving at high speed or immersed in a gravitational field. Even in relativity, however, time does not flow in reverse.”

According Kleban, general relativity is a theory of gravity and also a radical reformulation of the nature of space and time that establishes their intimate and dynamic connection. Thus, the apparent force of gravity ceases to be a common force and becomes something like a “pseudoforce” or “fictitious force.”

After detailing some implications of this scenario, including the curvature of space-time, Kleban explained why Hubble’s Law works: “It is because the universe is expanding, and this has implications for the past and for the future.”

With regard to the future, some believe that the expansion will continue infinitely; the speed of expansion will decrease gradually, but will never cease. According to this hypothesis, Kleban explained, stars will eventually consume all their fuel and the universe will become cold and dead, even though this would probably not be the end, which would occur later.

Other researchers think the expansion will reach a maximum level and the universe will then begin to contract. After a finite span of time, density will be infinite, a singularity that is called “Big Crunch” (major collapse).

There are also those who consider that a threshold situation is possible between these two scenarios.

However, Kleban explained, these hypotheses hold two surprises: the first is that over the last billion years the expansion of the universe has accelerated because of dark energy. The second is that the speed of the expansion seems to be very close to the threshold speed. This would mean that the universe will continue to expand forever and will never reach zero degree or undergo an actual “hot death.”

Multiverse

And how about the beginning of everything, the Big Bang? Kleban said, “some well-intentioned additions to the laws of physics can dramatically affect the nature of the Big Bang and remove the singularity without altering any experiment carried out on Earth.”

One of his main interests is the so-called “multiverse” of string theory. “In string theory, the Big Bang was not a singularity or the beginning of time. It was the birth of a ‘bubble’ of a new ‘phase’.” The multiverse could harbor the emergence of numerous such bubbles.

String theory allows us to understand what existed before the Big Bang and what exists “beyond the universe” (or rather, what exists outside the visible bubble where the observable part of the universe is inserted), according to Kleban. However, he cautioned that the theory does not work with regard to “big crunches” (due to the arrow of time, actually) that the theory itself envisages. “Likewise, the theory does not work in low entropy situations.”

In conclusion, Kleban said that, for him, the most attractive idea is an overall timeless universe, where almost all of time in a state of balance with near‑maximum entropy – only rare fluctuations of reduced entropy, which would produce a local arrow of time. However, “this idea does not seem to work, but rather predicts miracles.”

Photo: Sandra Sedini/IEA-USP