Sky Father seems to like to mess with our expectations. Not only has he left a surprise in our own solar system, made stars that are inexplicable and weird types of planets around other stars we would have never imagined, he also seems to be messing with the universe at large. We are just realizing he had stinkers waiting for us. We'll discuss one of them today.
First, let us take a step back. Cosmology is the study of the entire universe. This includes its origin, its current state and its ultimate fate. This might sound religious, but it is not. This is accomplished not by prayer and revelation, but through observation. Cosmology takes no stance on religion, but rather just observes, describes and predicts based on those actions how the universe is and will be.
Edwin Hubble, an American astronomer, noticed that certain stars went through predictable patterns of brightening and then fading. These were called the cepheid variables. These stars heat up, expand, cool down, and repeat in a clockwork fashion. Once he realized this, he noticed the light was redshifted enough indicate the stars were moving away. The universe, he discovered, was expanding. Prior to that point, it was assumed the universe was static and while not unchanging, was not contracting or expanding.
This led Belgian astronomer Georges Lemaître to suggest that if the universe was expanding, that if you tracked it back in time, the whole universe had originated in a single point. That original point, the singularity, and the massive expansion afterwards was described in a theory with the popular name of 'The Big Bang.' Just to be fair, Lemaître also proposed that the universe was expanding independent of Hubble.
All of that was one big stinker Sky Father let us have. But he was far, far from done.
There was much arguing over the fate of the universe after a lot of observations supported the Big Bang. Some said the universe would steadily expand forever. Others thought the universe would slow down and all the mass in the universe would cause the universe to reverse, collapse into itself and become a singularity again. This might then cause a Big Bang again.
It turns out none of these were right.
In the 1990s, Saul Perlmutter at Lawrence Berkeley National Lab in the US leading the Supernova Cosmology Project and an international team led by Brian Schmidt and Adam Riess called the High-Z Supernova Search Team were observing supernova, the massive explosions that take place when a large enough star dies. Both teams noticed that it appeared rather than just receding at a steady pace, the universe was expanding at an accelerating rate. Both teams published their results within weeks of each other, despite working very independently.
This is not at all what we expected.
Sky Father had yet another stinker for us.
Explaining the accelerating expansion of the universe is one of the preeminent problems in cosmology today. Why is this happening? What is causing it? What does it mean for the fate of the universe?
Something is causing the acceleration and overcoming gravity. That something has been dubbed 'dark energy.' Do not confuse dark energy with dark matter. They are very, very different things. However, dark energy is a subject where there has been an insane amount of arguing. The only thing really agreed on is that its a good label for describing something causing the universe to expand in an ever faster rate.
One model suggests dark energy is a form of energy that is inherent to the universe and it's been called the cosmological constant. Amusingly, Einstein originally proposed the universe had the constant flux of energy, but then changed his mind, calling it his biggest mistake ever. Sky Father apparently thought otherwise and got his giggle.
As for what will be the ultimate fate of the universe, that remains to be seen. Will dark energy cause all the separate galaxies to recede from one another so they cannot see one another anymore? Or will dark energy cause the space between everything, even locally, to grow so much that everything cannot exist as matter anymore? Will the universe eventually only or at least dominated by dark matter?
We don't know.
We will see. Or maybe one of our descendants might. If we last long enough to figure out the puzzle Sky Father left us. How many fingers more will he make us pull, do you think?
Either way, he's had his winking prank at least twice. There might be more.
