The expansion of the universe and the cooling down of the universe are two separate concepts that are not directly related to each other. Let's explore each concept:
- Expansion of the Universe: The observation that galaxies are moving apart from each other at high speeds is a consequence of the expansion of the universe. This phenomenon was first discovered by Edwin Hubble in the 1920s and is now well-established through various observational evidence.
According to the Big Bang theory, the universe started from an incredibly hot, dense, and small state about 13.8 billion years ago. Since then, it has been expanding, and galaxies have been moving away from each other. The expansion is not due to galaxies physically moving through space but rather the space between them stretching. This expansion is happening on a large scale, encompassing the entire universe.
It's important to note that the expansion of the universe does not imply that galaxies are moving through a preexisting space like objects moving apart in a room. Instead, the expansion itself creates more space between galaxies as time goes on.
- Cooling Down and Density: The cooling down of the universe refers to the decrease in temperature over time. In the early stages of the universe, shortly after the Big Bang, the universe was extremely hot and dense. As the universe expanded, it also cooled down.
However, the expansion of the universe does not necessarily mean that it is becoming less dense. The overall density of the universe depends on the balance between matter (including both ordinary matter and dark matter) and energy (including both normal energy and dark energy). While the density of matter decreases as the universe expands, the density of dark energy remains constant or even increases.
Dark energy is a mysterious form of energy that is thought to be responsible for the acceleration of the universe's expansion. It counteracts the gravitational pull of matter and can cause the expansion to accelerate over time. As a result, even though the density of matter decreases, the expansion can continue or even accelerate.
In summary, the expansion of the universe and the cooling down of the universe are distinct phenomena. The expansion is driven by dark energy, while the cooling down refers to the decrease in temperature over time. The expansion does not necessarily mean a decrease in density, as the influence of dark energy can counteract the dilution of matter.