How is the Universe Expanding?

[Alex K]

For years scientists said the universe was expanding after the Big Bang, but that the rate of expansion was slowing down under mutual gravitational attraction.
But more recently they're saying it's not slowing down, but that the expansion is speeding up!
Wish they'd make up their minds!
PS- and if it is speeding up, exactly what forces are at work?

The fundamental equations governing the expansion of space are derived directly from Einstein's field equations. One crucial input one has to insert into them in order to get a result is how the energy in the universe changes as the universe expands. There are three main types of "stuff" which behave differently in that respect. Matter simply gets diluted as the universe expands, so its energy per volume decreases as the volume increases. Next there is radiation and high energy particles, those get diluted and alsos redshifted as the universe expands. Last but not least, fields such as the Higgs field contribute a constant energy density per volume that doesn't change as the universe expands. These three types of energy content drive the expansion in different directions. Radiation and matter brake the expansion, whereas energy forms of the last type exert a negative pressure and make the universe expand more quickly. If only radiation and matter are present, einstein's equations yield a slowing expansion, but if the last kind of energyis present in significant quantities, this overcomes the braking effect of matter and radiation, resulting in an accelerated expansion. It was unknown whether that was the case, but around 1998, evidence from observations of supernova explosions came in which showed an accelerated expansion. This was evidence that the kind of constant non-diluting energy was also present in the universe. It was dubbed Dark Energy because it infuences the expansion doesn't show up in telescopes. In reality, Dark Energy is probably a mix of different contributions from the cosmological constant in the field equations, virtual particles and the higgs field. New evidence suggests that maybe there are dark energy factors that slowly change with time, but this is far from being settled.

It'a not like scientists change their minds all the time for no reason, they use occam's razor and follow the evidence - before ca 2000 there simply was no compelling evidence for accelerated expansion, so the absence of significant amounts of a constant energy was the most parsimonious assumption.