At 13.8 billion years ago, our entire observable universe was the size of a peach and had a temperature of over a trillion degrees.

That's a pretty simple, but very bold statement to make, and it's not a statement that's made lightly or easily. Indeed, even a hundred years ago, it would've sounded downright preposterous, but here we are, saying it like it's no big deal. But as with anything in science, simple statements like this are built from mountains of multiple independent lines of evidence that all point toward the same conclusion — in this case, the Big Bang, our model of the history of our universe.

But, as they say, don't take my word for it. Here are five pieces of evidence for the Big Bang:

Imagine for a moment that we lived in a perfectly infinite universe, both in time and space. The glittering collections of stars go on forever in every direction, and the universe simply always has been and always will be. That would mean wherever you looked in the sky — just pick a random direction and stare — you'd be bound to find a star out there, somewhere, at some distance. That's the inevitable result of an infinite universe.

And if that same universe has been around forever, then there's been plenty of time for light from that star, crawling through the cosmos at a relatively sluggish speed of c, to reach your eyeballs. Even the presence of any intervening dust wouldn't diminish the accumulated light from an infinity of stars spread out over an infinitely large cosmos.

Ergo, the sky should be ablaze with the combined light of a multitude of stars. Instead, it's mostly darkness. Emptiness. Void. Blackness. You know, space.

The German physicist Heinrich Olbers may not have been the first person to note this apparent paradox, but his name stuck to the idea: It's known as Olbers' paradox. The simple resolution? Either the universe is not infinite in size or it's not infinite in time. Or maybe it's neither.

As soon as researchers developed sensitive radio telescopes, in the 1950s and '60s, they noticed weirdly loud radio sources in the sky. Through significant astronomical sleuthing, the scientists determined that these quasi-stellar radio sources, or "quasars," were very distant but uncommonly bright, active galaxies.

What's most important for this discussion is the"very distant" part of that conclusion.

Because light takes time to travel from one place to another, we don't see stars and galaxies as they are now, but as they were thousands, millions or billions of years ago. That means that looking deeper into the universe is also looking deeper into the past. We see a lot of quasars in the distant cosmos, which means these objects were very common billions of years ago. But there are hardly any quasars in our local, up-to-date neighborhood. And they’re common enough in the far-away (that is, young) universe that we should see a lot more in our vicinity.

We live in an expanding universe. On average, galaxies are getting farther away from all other galaxies. Sure, some small local collisions happen from leftover gravitational interactions, like how the Milky Way is going to collide with Andromeda in a few billion years. But at large scales, this simple, expansionary relationship holds true. This is what astronomer Edwin Hubble discovered in the early 20th century, soon after finding that "galaxies" were actually a thing.

In an expanding universe, the rules are simple. Every galaxy is receding from (almost) every other galaxy. Light from distant galaxies will get redshifted — the wavelengths of light they're releasing will get longer, and thus redder, from the perspective of other galaxies. You might be tempted to think that this is due to the motion of individual galaxies speeding around the universe, but the math doesn’t add up.

The amount of redshift for a specific galaxy is related to how far away it is. Closer galaxies will get a certain amount of redshifting. A galaxy twice as far away will get twice that redshift. Four times the distance? That's right, four times the redshift. To explain this with just galaxies zipping around, there has to be a really odd conspiracy where all the galactic citizens of the universe agree to move in this very specific pattern.

Instead, there's a far simpler explanation: The motion of galaxies is due to the stretching of space between those galaxies.

We live in a dynamic, evolving universe. It was smaller in the past and will be bigger in the future.

Let's play a game. Assume the universe was smaller in the past. That means it would have been both denser and hotter, right? Right — all the content of the cosmos would've been bundled up in a smaller space, and higher densities mean higher temperatures.

At some point, when the universe was, say, a million times smaller than it is now, everything would have been so smashed together that it would be a plasma. In that state, electrons would be unbound from their nuclear hosts and free to swim, all of that matter bathed in intense, high-energy radiation.

But as that infant universe expanded, it would've cooled to a point where, suddenly, electrons could settle comfortably around nuclei, making the first complete atoms of hydrogen and helium. At that moment, the crazy-intense radiation would roam unhindered through the newly thin and transparent universe. And as that universe expanded, light that started out literally white-hot would've cooled, cooled, cooled to a bare few degrees above absolute zero, putting the wavelengths firmly in the microwave range.

Push the clock back even further than the formation of the cosmic microwave background, and at some point, things are so intense, so crazy that not even protons and neutrons exist. It's just a soup of their fundamental parts, the quarks and gluons. But again, as the universe expanded and cooled from the frenetic first few minutes of its existence, the lightest nuclei, like hydrogen and helium, congealed and formed.

We have a pretty decent handle on nuclear physics nowadays, and we can use that knowledge to predict the relative amount of the lightest elements in our universe. The prediction: That congealing soup should have spawned roughly three-fourths hydrogen, one-fourth helium and a smattering of "other."

The challenge then goes to the astronomers, and what do they find? A universe composed of, roughly, three-fourths hydrogen, one-fourth helium and a smaller percentage of "other." Bingo.

There's more evidence, too, of course. But this is just the starting point for our modern Big Bang picture of the cosmos. Multiple independent lines of evidence all point to the same conclusion: Our universe is around 13.8 billion years old, and at one time, it was the size of a peach and had a temperature of over a trillion degrees.

In other news...

Unsolved problems in the Big Bang model

The are few unsolved issues in the standard Big Bang model. These are as follows:

1. Horizon problem: There are portions of the universe that are visible to us but invisible to each other. Horizon problem points out that different region of the universe have not yet contacted each other due to the great distances between them, but nevertheless they have the same temperatures and other physical properties. We know that CMBR is found to be homogeneous everywhere. How it became possible? The observed isotropy of the CMB is the problem in this regard. Because we believe that information cannot travel faster than light. The resolution of this apparent inconsistency is offered by inflationary theory in which a homogeneous and isotropic scalar energy field dominates the universe at some very early period. According to Heisenberg, during the inflationary phase, there was a ‘Quantum thermal fluctuations’ which would be magnified to cosmic scale. These fluctuations serves as the seeds of all current structure in the universe. Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian, which has been accurately confirmed by the measurement of the CMBR.

The instant before inflation began, universe was only about $10^{-24}cm$ in diameter. All matter and energy were in close and uniform contact within the briefest instant, the universe expanded exponentially by a factor of about $10^{50}$ , stretching once intimately connected matter and energy to the farthest reaches of the universe. The information contained in the per-inflationary universe didn’t have to travel the speed of light, it traveled at the speed of inflation.

2. Flatness problem: According to Einstein field equations of general relativity, the structure of space-time is affected by the presence of matter and energy on small scales, space appears flat – as does the surface of the Earth if one looks at a small area. On large scale, space is bent by the gravitational effect of matter. The amount of bending (or curvature) of the universe depends on the density or matter/energy present. According to cosmology (Friedman – Lemaitre – Robotson – Waker metric), the universe may have positive, negative or zero spatial curvature depending on its total energy density (k).

Curvature is negative if k<0 (hyperbolic)
Curvature is positive if k>0 (spherical)
Curvature is flat if k=0 (flat)

Total energy density is a fine tuned parameter between the density of matter and energy in the universe. The values of total energy density departs rapidly from the critical value over cosmic time.

Now, the question is, during Big Bang nucleo-synthesis, what was the values of this parameter? Positive or negative or zero? How it was so fine tuned? We know that the reality that our universe is approximately flat. Thus, the value must be extremely close to ‘One’ (in 1 / 64 th) i.e. $\Omega_o$ initially must have almost exactly the number given below which is extremely close to one. 1.0000000000000000000000000000000000000000000000000000000000001

There is no known reason for the density of the universe to be so close to the critical density, this appears to be an unacceptably strange coincidence in the view of most astronomers.

Many attempts have been made to explain the flatness problem. Modern theory includes the idea of inflation which predicts the observed flatness of the universe. A brief period of extremely rapid expansion maintained the situation of flatness. Because, before the expansion (i.e. inflation) all matter and energy were intimately connected. At that time, the density is very close to (fine tuned) the critical density of the universe.

3. Magnetic monopole problem: Monopole is a hypothetical particle in physics that is a magnet with only one pole and it will have a net magnetic charge. In the year 1931, Dirac proposed quantum theory of magnetic charge. In his theory, he showed that the existence of monopole was consistent with Maxwell equations only if electric charges are quantized, which is experimentally observed. Since then, several systematic monopole searches have been performed. Now, we know that the monopole detection problem is an open problem in experimental physics.

The grand unification theory (GUT) and superstring theory (both theories successfully combine strong and electro-weak force) predicts the existence of magnetic monopole. According to GUT, the topological defects in space is termed as magnetic monopole. These defects were produced efficiently in the hot early universe, resulting in a density much higher than observed. No monopole is observed till date. It is believed that the inflation removed all topological defects from the observable universe. Thus, inflation drives the geometry to flatness, inflation maintains isotropy and inflation removes all point defects.

4. Baryon asymmetry: As we discussed in the Big Bang theory that an unknown process called Baryogenesis created the asymmetry. For baryogenesis to occur, Sakharov conditions must be satisfied. These require that Baryon number is not conserved, that c and cp- symmetry are violated and that the universe depart from thermodynamic equilibrium. All these conditions occur in the standard model, but the effect is not strong enough to explain the present baryon asymmetry. Baryon asymmetry lead the dominance of matter over antimatter.

5. Dark matter: Numerous observations (anisotropies in the CMB, galaxy cluster velocity dispersions, large scale structure distributions, gravitational lensing studies and x-ray measurement of galaxy clusters) have indicated the existence of dark matter. The rotation curve of galaxies hint that the dark matter particle exists in the halo of the galaxies. So, the dark matter is the reality. However, dark matter particles have not been observed in laboratories. Many candidates for dark matter have been proposed and several projects are underway. The standard big bang model have not explained the existence of dark matter particles.

6. Dark matter: Measurement of red shift magnitude relation for supernova indicate that the expansion of the universe has been accelerating since the universe was about half its present age. To explain this acceleration, general relativity requires ‘negative pressure’, called Dark (or Vacuum) energy. Negative pressure is a property of vacuum energy, but the exact nature of dark matter remains one of the great mystries of the Big Bang. Possible candidates include ‘Cosmological constant’ and ‘quintessence’ (hypothetical form of dark energy postulated as an explanation of observations of accelerating universe and is a scalar field). Results from the WMAP (2008) indicate that the universe today is 73% dark energy, 23% dark matter and 4.6% regular matter and less than 1% neutrinos.

“At 13.8 billion years ago, our entire observable universe was the size of a peach and had a temperature of over a trillion degrees.”

There were peaches 13.8 billions years ago? Who knew?

And this is observable.

“Magnetic monopole problem: Monopole is a hypothetical particle in physics that is a magnet with only one pole and it will have a net magnetic charge. In the year 1931, Dirac proposed quantum theory of magnetic charge. In his theory, he showed that the existence of monopole was consistent with Maxwell equations only if electric charges are quantized, which is experimentally observed. Since then, several systematic monopole searches have been performed. Now, we know that the monopole detection problem is an open problem in experimental physics.”

It’s a problem when people can’t see the forest for the trees. Imagine an arrangement of bi-polar magnets in a spherical formation in which a pole is directed toward the center and the other pole is directed outward from the center. And these magnets are so tightly placed together so as to disallow magnetic flux to flow backwards between them. (This is not physically possible, but as a thought experiment it is useful.) You would have a “monopole permanent magnet”. How would it behave?

Simple question: based on the observable phenomenon of Fleming’s right-hand and left-hand rules, what are the implications for motion, field, and current of such a magnetic monopole?

You will find the ordinary behavior of a photon electromagnetically is similar to what we expect from a so-called magnetic monopole. Technically the theoretical magnetic monopole is a magnetic photon. But photons already have magnetic properties. But a magnetic monopole is defined as different from the behavior of “normal” photons which pair with leptons.

I think a photon IS a magnetic monopole which scientists simply want to make stand still and be examined in a moment of time. But photons don’t cooperate that way.

Again. Forest. Trees.

Dirac’s quantization proof supports that photons already are magnetic monopoles. The real question is why magnetic monopoles do not follow the left- and right-hand rules due to pairing with leptons.

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.