Our Universe Part 14: The End of the Universe

The universe has been expanding since the Big Bang and its rate of expansion is increasing. Some mysterious component of the universe is opposing gravitational attraction and it's called dark energy. This discovery puts popular notions about the fate of the universe into question.

The discovery of cosmic background radiation in the 1960's gives tremendous support for the Big Bang theory, so much that it is now one of the most firmly rooted models in astrophysics. It also gave physicists a framework in which to measure how old the universe is, about 13.7 billion years old. This theory began as a series of solutions to Einstein's general relativity equations carried out in 1912 by Alexander Friedman. The implications of those solutions created a lot of controversy at the time. Einstein himself could not accept that the universe is not static and eternal, so he added a cosmological constant to his equations, a decision he later regretted because at the same time American astronomer Vesto Slipher discovered that all the galaxies he observed were redshifted, and thus, moving away. In 1924, Edwin Hubble confirmed these observations with his own. In 1927, the Catholic priest and physicist, Georges Lemaitre, confirmed Friedman's calculations and extrapolated backward to a point of infinite temperature and density at a particular time in the past, He called this infinite point a "primeval atom." This was not without religious controversy - a scientifically defined moment of creation! Lemaitre is sometimes credited with coining the term Big Bang but it was Fred Hoyle, who was promoting his own competing Steady State theory, who first used the term (derisively) during a 1950's radio broadcast (although he denied this). Here is what the Big Bang theory looks like (in a flat universe for simplicity):

Since the 1920's the Big Bang theory has been supported and refined by data from COBE, the Hubble Space Telescope and WMAP, as well as by the unexpected discovery that the rate of expansion is increasing (this will be explored shortly).

Questions about the workings of the universe still remain however. A number of them, the flatness problem, the magnetic monopole problem and the horizon problem, have all been more or less resolved by cosmic inflation. But baryon asymmetry (why there is more matter than antimatter) is not resolved. 

The discovery that the rate of expansion is increasing brings us to the recently formed concept of dark energy. In the late 1990's astronomers were surprised by the magnitude of redshift for type 1a supernovae. These supernovae are used as a standard candle in astronomy because they all exhibit nearly identical luminosities. They are the result of white dwarf explosions, and white dwarf stars are all confined to a very narrow mass limit called the Chandrasekhar limit. White dwarf stars are covered in my article, Introduction to the Stars. They can be used to measure both distance and redshift and because almost 97% of all stars in the Milky Way alone are destined to become white dwarfs (and then type 1a supernovae), so a lot of accurate measurements can be made. A roughly linear relationship between distance and redshift for galaxies was found, shown below, and this means that the expansion rate is increasing. But why?

(Brews ohare;Wikipedia)

Redshift velocity is plotted against distance, showing a roughly linear relationship, called Hubble's law. The Virgo Cluster of stars shows significant scatter because its redshift measurement is affected by other  effects, such as gravitational effects from other nearby galaxies. This is different from the Doppler effect, especially at higher values of z. It is a measurement of the recessional velocity of the galaxy due to the expansion of the universe. At zero z (the current universe), however, the Hubble law value and Doppler effect value are the same.

General relativity requires that, for the expansion rate to increase, much of the energy of the universe must come from a component that exhibits great negative pressure. This negative pressure has been called dark energy. Dark energy also explains why the universe is almost perfectly spatially flat (evidence for this is in the map of the cosmic background radiation), To be flat, the universe must maintain almost exactly the critical density for mass/energy. If it went out of whack even a tiny bit at any time during expansion, the universe would be either positively or negatively curved. The mass density of the universe has been measured and it's only 30% of critical density. Dark energy nicely accounts for the rest that is needed. Negative pressure is a property of vacuum energy but other than that, nothing is known about dark energy, except that while the energy density in matter is decreasing as the universe expands, the dark energy density remains constant. This means that the effect of dark energy should become more dominant in the future and the rate of the acceleration of the universe's expansion will continue to increase. Dark energy weeds out two possible universe fates: the Big Freeze and the Big Crunch. 

If the mass/energy density of the universe is below critical density, the expansion slows but never stops. Stars would gradually burn out and black holes would grow as they eventually collide with and swallow up the star remnants. The temperature of the universe would approach absolute zero and even black holes would disappear as they eventually evaporate away via Hawking radiation (explored in my article, Black Holes). This is the Big Freeze. It's also known as Heat Death. This 7-minute video paints a vivid picture of how the universe might end:



If the mass/energy density of the universe is above critical density, the universe would reach a maximum size and then contract, becoming hotter again until it perhaps re-exploded in a new Big Bang. This is the Big Crunch scenario.

With dark energy, more and more of the universe will pass beyond the visible boundary, possibly ending in what is coined the Big Rip. In this scenario, galaxies, stars, planets, all matter, will eventually be ripped apart. This is how it works: The size of the observable universe is shrinking as the rate of expansion increases because the distance to the observable edge, where everything is moving away at the speed of light, must get closer and closer. Eventually the observable (below the speed of light) size will contract to a size smaller than atoms or even subatomic particles. At this point none of the forces that hold particles together can act and the particles themselves are ripped apart. This 5-minute video explores the Big Rip possibility:

There are some problems with dark energy however. One is that while quantum field theory allows for such a force, it predicts that its density may be as much as 120 times greater than any calculated value based on astronomical observations. This is called the cosmological constant problem and it brings us back to Einstein's modification to general relativity in order to preserve his idea of a constant universe. He abandoned it back in the early 1900's but the recent discovery of accelerating expansion has brought it back out of the tool box. Physicists now think that the cosmological constant does not lead to a static universe in equilibrium because the equilibrium itself is unstable. As the universe expands, the expansion releases vacuum energy and that causes more expansion. This is a nutshell answer. If you would like to explore this interesting cosmological constant puzzle (and, really, a dark energy puzzle) in more detail, try this slide-show illustrating where the science is at right now.

There is also an idea that combines string theory and dark matter and it places our universe in a fascinating new context. String theory mathematics allow for dark energy, while leaving the value for the cosmological constant unassigned. When these calculations are done, as they were by Andrei Linde et al. in 2003, a graph is produced that is shaped like a mountainous landscape, where altitude represents the value of the cosmological constant. After the Big Bang, that value should settle on a low point of minimum energy. The catch here is that there are billions of possible low points to choose from and no obvious reason why the universe should have picked one value over another. Some physicists such as Leonard Susskind have embraced this and postulated a multiverse scenario in which there are countless universes coexisting at once, each with its own cosmological constant, whereas others point to these calculations as flawed because they can't make useful predictions. 

The idea of there being not just one universe but countless universes also finds some support from other areas of physics research, such as quantum mechanics.  For example, Richard Feynman popularized QED theory to explain the strange behaviours of photons. We experience a photon as travelling in a perfectly straight line from A to B. According to this theory, what we see is the sum of the greatest probability of its travel plan. In reality the photon travels through many multiple paths simultaneously (in a quantum state), with some of its personas even interfering with other personas in the process, to get from A to B. It is anti-intuitive but backed up by experiment, if you remember Young's famous double slit experiment where both the wave nature and particle nature of light are revealed. When a beam of photons is shot through a double slit barrier, an interference pattern shows up, just as you would expect waves to interact. When the beam is reduced to one photon shot at a time, an interference pattern still shows up. How do individual photons know where the next photon will strike? Even when photons are supposedly behaving as particles their wave nature still shows up (this experiment has been repeated with electrons showing that matter too exhibits what is called wave/particle duality). This experiment also hints at the quantum electrodynamic nature of the photon. The question becomes, is the universe a quantum system, exploring all possible physical constant values?

In 2006, Stephen Hawking and Thomas Hertog suggested that we view the universe as a quantum system, within the framework of string theory. In this case, instead of a photon, the universe itself follows all possible trajectories simultaneously as it evolves and moves forward through time. These are not multiple Susskind-like universes; they only exist in a quantum sense just as individual photon trajectories never exist in the classical sense. What exists in the classical sense, what we experience, is the maximum probability of trajectory possibilities. This is not without its difficulties. For example, how do we test the theory? One thing theorists can do is take our present universe and trace it backwards through time. There should be multiple possible branches the universe could have taken at any point, but they can weed out those that are too different from our universe. For example we know that our universe is almost perfectly flat so we can ignore all possible trajectories that rely on a cosmological constant that leads to curvatures. They could also test these results against possible universes that produce the cosmic background radiation we observe, and so on.

The fate of the universe ultimately relies on dark energy. But when physicists try to understand it, they run into a variety of problems, most of which have to do with assigning values to the cosmological constant, and that has lead researchers in a variety of directions and into notions of multiple universes. Understanding how dark energy works will be key to understanding how the universe will eventually end up.

One thing, however, seems fairly certain: dark energy's effects are growing evermore significant over time as gravity's effect's dwindle with decreasing energy/matter density in the universe. For now, this idea assumes we are heading for some kind of Big Rip, but no one knows for sure. That means that the universe's ultimate fate must remain a mystery, at least for now.