What is dark energy? Exactly what is dark energy?
Astrophysicists would also like to know the answer to this
question – it determines how the Universe will develop.
Cosmologists are fairly sure that it has been expanding since
the Big Bang. (For more on this topic, see the astronomy
question from week 38: How quickly is the Universe expanding?)
What is still uncertain is whether this expansion will continue
forever or whether the Universe will one day begin to collapse
again, and also how much mass is present in the Universe – or
would have to be present – to prevent one or other of these two
scenarios from occurring. When Albert Einstein formulated the
general theory of relativity at the beginning of the 20th
century, he assumed that the Universe is static. He therefore
had to include an additional expression in his equations,
referred to as the cosmological constant. In 1929, Edwin Hubble
discovered that the rate at which distant galaxies move away
from Earth increases the further away they are; this observ