What are two pieces of scientific evidence that supports the big bang theory?

The big bang is a widely accepted starting point for the universe. Experimental evidence strongly supports the evolution of the universe from a single hyper dense point. The first piece of experimental data to support the big bang model of cosmology is the existance of the cosmic microwave background (CMB). When the universe was first created a huge amount of energy was released. This energy formed into quarks, leptons over time. The quarks combined to form protons. The formation of atoms did not occur at this point as the electrons were unable to combine with the protons due to extremely high temperatures. An additional consequence of these temepratures was a vast amount of light was produced. This light bounced around inside the primordial soup of plasma (electron free protons) unable to escape due to the high density of protons. As the universe expanded and cooled the electrons eventually combined with the protons (and He nuclei that had also formed over the 360,000 years of plamarised universe) to form the first atoms.

Once this had occured the photons were able to escape. As a consequence of the sheer scale of the universe and the finite speed of light some of these photons are only reaching our detectors now, they took that long to cross the void of the universe. It has been found that the energy of these photons is roughly uniform in all directions of the sky. This suggests they must have formed at every point in the universe at the same time. This experimental conclusion matches the theoretical explanation above, it is the most reasonable explanation of this phenomena. Tiny changes in the energy of the photons from different parts of the sky also correspond to where galaxies form, supporting the current model for galaxy formation, which also stipulates that there was a big bang.

The next piece of experimental data comes from the expansion of the galaxy. The galaxy is expanding uniformly in all directions. Initially this placed our galaxy in the centre of the universe, it was thought everything was receeding from us. However, the way superclusters (groups of groups of galaxies) interact showed diminishing gravitational effects. Since gravity weakens as separation increases this suggested that each cluster was also receeding from each other. This implies that every group of galaxies is moving away from every other group of galaxies. The easiest way to understand this is to imagine drawing dots on a baloon. When you blow up the baloon every point moves away from one another but none remain stationary. Just as with th baloon this means that everything was once closer together. If this is extrapolated then at some time everything must have been at the same spatial position, thrust out by the inital big bang.