Universe (2)

Surprisingly, we know quite a lot about the early history of the Universe. In particular, the period starting approximately one decillionth (1×10-33) of a second after Big Bang is almost fully understood. We know exactly what happened with the quark “soup”, how the particles and anti-particles annihilated, when the neutrinos were released etc. Before the one decillionth of a second there most likely was a period of “cosmic inflation” during which a tiny volume of space expanded insanely fast for a very short period of time. There is a lot of supporting evidence and few problems with the inflation theory so it is generally accepted by the astrophysicists as correct. Going further back in time we are hitting the “Planck time” after Big Bang – the smallest measurable time interval we can meaningfully think about. This is where the known laws of physics break down and we currently have no tools to examine what had happened prior.

At approximately 400 000 years after Big Bang the plasma in the entire Universe became transparent to visible light and photons raced off like crazy in all directions. As a result of space expansion (more on that later) these photons have since red-shifted and can now be detected in the microwave range as “cosmic radiation”. There are tiny inhomogeneities in the spatial distribution of this radiation which means that the Universe at 400 000 years was already lumpy. To be able to look further back in time we would need to detect the spatial distribution of neutrinos which raced off only one second after Big Bang. We have no technical means to do it at present so the oldest image of the Universe we have is 400 000 years after Big Bang.

The lumpiness of the Universe we can observe in this image must have been present a lot earlier – probably already at the moment of Big Bang. One explanation of these inhomogeneities is that they are quantum fluctuations of an incredibly tiny volume of space which, for some reason, decided to start expanding to become the Universe we know. The smallest distance we can meaningfully talk about is the so called Plank length – a cousin of Planck time. There are indications that the Universe started as a volume of space some 100-150 Planck lengths across. The energy propelling the initial burst of expansion 13.6 billion years ago could have been provided by the space itself (so called “vacuum energy”). It is not quite clear why the expansion has kept going since but we have some clues.

As a result of various observations made in the 1990s we now know that the Universe is composed of 5% “ordinary” matter, 25% “dark” matter and 70% “dark” energy. There are also tiny contributions form neutrinos and radiation. Because of E=MC2 the energy is equivalent to and a form of matter so the two can be tallied together. Dark matter is invisible to us but we can deduce its existence from the gravitational behaviour of the galaxies (speed of rotation, light lensing etc). It is likely to be composed of WIMPs – weakly interacting massive particles – which are incredibly hard to detect. Dark matter contributes to the gravity pull pervading the Universe. We have no idea what dark energy is except that it repulses space. As space expands the amount of dark energy in it grows. The more space expands the more dark energy, the more repulsion, the more expansion – you get the picture?

Now the interesting bit. Dark energy repulses mass and directly counteracts the gravity pull. The Universe has always been a scene of this tug-of-war and actually the outcome was hard-coded in the balance between the amount of mass and speed of expansion at the end of cosmic inflation. It is like throwing a ball up in the air. It will normally be pulled back by gravity but if thrown hard enough it will overcome the gravity of Earth and scoot off into space. The outcome is already known the moment the ball is being released.

First the bad news. At 9.5 billion years old the Universe reached the point of no return and the space is now a runaway train propelled by dark energy. The initial speed of expansion was too much for the mass to arrest it. This expansion of space will accelerate forever, unless dark energy at some point stops its repulsive influence. The good news is that pockets of increased density of mass create enough pull to resist this universal expansion. All objects up to the galaxy cluster level have enough mass to not be ripped apart by the space expansion. Galaxy superclusters though will become separated and will fall outside one another’s event horizon. So the night sky of the future will have less and less stars as the galaxies in other superclusters are carried away by expanding space at faster than speed of light and we lose any contact with them.

One more thing – the image is that of space expanding, not galaxies moving through space. This is why speed of light is not the limit. In fact most of the Universe is already moving away from us faster than light which is why we cannot see it!

Universe (3)


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