Knocking on Heaven's Door - Lisa Randall [64]
The discovery of dark energy was the most profound physics wake-up call of the late twentieth century. Although there is much we don’t yet know about the evolution of the universe, we have a spectacularly successful understanding of the universe’s evolution based on the so-called Big Bang theory supplemented by a period of exponential expansion of the universe known as cosmological inflation.
This theory has agreed with a range of observations, including observations of the microwave radiation in the sky—the microwave background radiation left over from the time of the Big Bang. Originally the universe was a hot dense fireball. But during the 13.75 billion years of its existence it has diluted and cooled substantially, leaving this much cooler radiation that is a mere 2.7 degrees kelvin today—only a few degrees Celsius above absolute zero. Other evidence for the Big Bang theory of expansion can be found in detailed studies of the abundances of nuclei that were made during the universe’s early evolution and in measurements of the universe’s expansion itself.
The underlying equations we use to figure out how the universe evolves are the equations Einstein developed in the early twentieth century that tell us how to derive the gravitational field from a given distribution of matter or energy. These equations apply to the gravitational field between the Earth and the Sun but they also apply to the universe as a whole. In all cases, in order to derive the consequences of these equations, we need to know the matter and energy that surround us.
The shocking observation was that measurements of the characteristics of the universe required the presence of this new form of energy that is not carried by matter. This energy is not carried by particles or other stuff, and it doesn’t clump like conventional matter. It doesn’t dilute as the universe expands but maintains a constant density. The expansion of the universe is slowly accelerating as a consequence of this mysterious energy, which resides throughout the universe, even if it were empty of matter.
Einstein had originally proposed such a form of energy in what he called the universal constant, which later became known to physicists as the cosmological constant. Shortly after, he thought it a mistake and, indeed, that his use of it to try to explain why the universe was static was misguided. The universe does in fact expand, as Edwin Hubble showed soon after Einstein proposed the idea. The expansion is not only real, but it now seems that its current acceleration is due to the funny type of energy that Einstein had introduced and quickly dismissed in the 1930s.
We want to understand this mysterious dark energy better. Observations at this point are designed to determine whether it is just the sort of background energy that Einstein first proposed or whether it is a new form of energy that changes with time. Or is it something entirely unanticipated that we don’t yet even know how to think about?
OTHER COSMOLOGICAL INVESTIGATIONS
This is only a sampling—albeit an important one—of what we are now investigating. In addition to what I have already described, many more cosmological investigations are in store. Gravity wave detectors will look for gravitational radiation from merging black holes and other exciting phenomena involving large amounts of mass and energy. Cosmic microwave experiments will tell us more about inflation. Cosmic ray searches will tell us new details about the content of the universe. And infrared radiation detectors could find new exotic objects in the sky.
In some cases, we will understand the observations sufficiently well to know what they imply about the underlying nature of matter and physical laws. In other cases, we’ll spend a lot of time unraveling the implications. Regardless of what happens, the interplay between theory and data will lead us to loftier interpretations