Hubble theorized that the light we can see from distant objects in space appears different colors depending on whether that object is moving closer to or further from the earth. This principle, called redshift, is not complex.

A wave emitted by a source, which is moving with respect to an observer, will have a changed frequency when observed. This is called the Doppler effect. For relative motion, where the distance between the source and the receiver is increased, the waves will have longer wavelengths, and the opposite is true if the distance is decreased.

The classic example is the sound of an ambulance siren. As the ambulance approaches you, the sound waves are compressed and are shorter. This will give the siren a higher pitch. However, as the ambulance travels away from you, the waves lengthen, lowering the pitch of the siren.

Astronomers examining the patterns of lines emitted by distant stars and galaxies noted that the light was occasionally slightly blue, or more often, slightly red. This implies that the stars with a blueshift are moving toward the earth and those with a redshift are moving away from the earth.

This video from BBC helps us understand what many scientists believe about redshift:

Redshift prompted Hubble to conclude that the universe is expanding. However, while it is clear that there are redshifts in the light spectra of space objects, there is room for other interpretations of that redshift. Tangential, not just radial, velocities can produce redshifts, but there is no way of measuring tangential velocities for distant galaxies.ⁱ

According to Keith Wanser, who is professor of physics, California State University, Fullerton, other models for the formation of the universe offer acceptable alternatives to the Big Bang theory. Alternative theories have been proposed such as "white hole" cosmology, a recent creation of the earth, a bounded universe, an initial water mass, and Einstein’s theory of relativity.ⁱⁱ

The Big Bang theory proposes that the universe started with extremely high temperatures and cooled as it expanded. When the temperature had decreased after some 300,000 years to 3000K, matter and radiation became decoupled.

In the early hot stages, matter and energy could freely change places with each other because they were coupled. The high radiation would then be free to hurtle into space. However, since it is theorized that space itself is expanding, the temperature of the photons of energy falls until they have the frequency of microwaves. The background temperature of the Universe dropped to a mere 3K, and this radiation is called the echo of the Big Bang or the 3K microwave background radiation (MBR).

Here is one explanation of background radiation:

A NASA map of cosmic microwave background radiation.
Credit: NASA, DMR, COBE Project Public Domain...

With a traditional optical telescope, the space between stars and galaxies (the background) is completely dark. But a sufficiently sensitive radio telescope shows a faint background glow, almost exactly the same in all directions, that is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the radio spectrum...

Cosmic background radiation is well explained as radiation left over from an early stage in the development of the universe, and its discovery is considered a landmark test of the Big Bang model of the universe. When the universe was young, before the formation of stars and planets, it was smaller, much hotter, and filled with a uniform glow from its white-hot fog of hydrogen plasma. As the universe expanded, both the plasma and the radiation filling it grew cooler. When the universe cooled enough, stable atoms could form. These atoms could no longer absorb the thermal radiation, and the universe became transparent instead of being an opaque fog. The photons that existed at that time have been propagating ever since, though growing fainter and less energetic, since exactly the same photons fill a larger and larger universe. This is the source for the alternate term relic radiation.ⁱⁱⁱ

Actual measurements, however, show numerous inconsistencies in these assumptions, and the temperature speculations are also problematic.

The high temperatures at the beginning would prevent gas clouds—and thus, stars—from forming. A cloud of gas at a high temperature tends to fly apart, but the theory requires the gas to condense into stars. The gravitational forces between the gas molecules thus need to be great enough to allow for such contraction.

Calculations show that unless the temperature is less than 5K, the thermal energy of the cloud will tend to make the cloud expand and gravitational forces will be too low for contraction. Today, in the clouds where it is suggested that stars are forming, the temperatures are far too high for this to take place, but the during the Big Bang they would have been even higher. It thus seems even more unlikely for them to have contracted in the past than it is today.

Orion's hydrogen winds.
CC BY-NC 2.0 Adam Evans https://www.flickr.com/photos/astroporn/5152305006/in/album-72157624497531696/...

The Big Bang theory does not allow for any other atomic elements to have originated from the explosion than hydrogen, helium, lithium, and beryllium. How, then, did any other matter matter originate? This is one of the major problems with the Big Bang theory.

It is speculated that the Big Bang originated from a quantum fluctuation of the vacuum, but experimental data shows that when particles are brought into existence from energy, something called the Baryon number is conserved. This means that when these particles are produced, they are produced in equal numbers of matter and antimatter. For every electron there is thus a positron and for every proton, there is an antiproton.

This symmetry would also have been the case in the Big Bang. However, complete annihilation of both the matter and antimatter would have occurred, and the universe would have consisted of radiation only.

The universe, however, has a complete dominance of matter over antimatter, or else we would not be here.

Because of this problem, the Grand Unified Theories (GUT) have been proposed by physicists in attempts to solve the problem mathematically. These theories violate Baryon number conservation, and in turn predict proton decay, which has not been observed experimentally.

Once again, we see how ad hoc theories have to be proposed to prop up the standard model when the data does not fit the predictions. No wonder John Maddox, editor of the journal Nature, said this:

In all respects save that of convenience, this view of origin of the universe is thoroughly unsatisfactory.^iv