Heavier nuclei could now survive the gamma-ray bombardment. Primordial nucleosynthesis kicked in, enabling nuclear forces to bind protons and neutrons together, until the expansion of the universe made it too cold for these fusion reactions to continue.
In these 20 minutes, the universe was populated with atoms. The resulting elemental composition of the universe weighed in at roughly 76 percent hydrogen, 24 percent helium, and trace amounts of lithium—all ionized, since it was too hot for electrons to stably orbit these nuclei.
And that was it, until the first stars formed and began to forge all the other elements of the periodic table. Before these stars could form, however, newly-formed hydrogen and helium atoms needed to gather together to make dense clouds.
These clouds would have been produced when slightly denser regions of the universe gravitationally attracted matter from their surroundings. The question is, was the early universe clumpy enough for this to have happened? To answer the question, we can look to the modern-day night sky.
In it, we see a faint glow of microwave radiation that has an even fainter pattern in it. At that time, the universe had just cooled to about 3, Kelvin. Free electrons started to be captured into orbit around protons, forming neutral hydrogen atoms. Photons from the flash of the Big Bang, whose progress had been impeded by their scattering off of unbound electrons, could now finally stream throughout the cosmos, essentially free.
These photons continue to permeate the universe today, at a frigid temperature of only 2. These sky maps suggested something surprising: The intensity of the residual heat from the Big Bang made the early universe too smooth for gas clouds to form.
Enter dark matter. Because it does not interact directly with light, it was unaffected by the same radiation that smoothed out ordinary matter.
Therefore it was left with a relatively high degree of clumpiness. It, rather than regular matter, initiated the formation of the stars and galaxies that make up the modern structure of the universe. All matter in the universe was formed in one explosive event In the American astronomer Edwin Hubble discovered that the distances to far-away galaxies were proportional to their redshifts.
Redshift occurs when a light source moves away from its observer: the light's apparent wavelength is stretched via the Doppler effect towards the red part of the spectrum.
If galaxies are moving away from us, reasoned Hubble, then at some time in the past, they must have been clustered close together. Subsequent calculations have dated this Big Bang to approximately In two teams of astronomers working independently at Berkeley, California observed that supernovae — exploding stars — were moving away from Earth at an accelerating rate.
In the 17 th and 18 th centuries CE , several key events helped revive the theory that matter was made of small, indivisible particles. In , Evangelista Torricelli , an Italian mathematician and pupil of Galileo, showed that air had weight and was capable of pushing down on a column of liquid mercury thus inventing the barometer.
This was a startling finding. If air — this substance that we could not see, feel, or smell — had weight, it must be made of something physical. But how could something have a physical presence, yet not respond to human touch or sight? Daniel Bernoulli , a Swiss mathematician, proposed an answer.
He developed a theory that air and other gases consist of tiny particles that are too small to be seen, and are loosely packed in an empty volume of space. The particles could not be felt because unlike a solid stone wall that does not move, the tiny particles move aside when a human hand or body moves through them. Bernoulli reasoned that if these particles were not in constant motion, they would settle to the ground like dust particles; therefore, he pictured air and other gases as loose collections of tiny billiard-ball-like particles that are continuously moving around and bouncing off one another.
Many scientists were busy studying the natural world at this time. Shortly after Bernoulli proposed his theory , the Englishman Joseph Priestley began to experiment with red mercury calx in Mercury calx, a red solid stone, had been known and coveted for thousands of years because when it is heated, it appears to turn into mercury, a silver liquid metal.
Priestley had observed that it does not just turn into mercury; it actually breaks down into two substances when it is heated, liquid mercury and a strange gas.
Priestley carefully collected this gas in glass jars and studied it. After many long days and nights in the laboratory, Priestley said of the strange gas, "What surprised me more than I can well express was that a candle burned in this air with a remarkably vigorous flame.
Priestley's discovery revealed that substances could combine together or break apart to form new substances with different properties. For example, a colorless, odorless gas could combine with mercury, a silver metal, to form mercury calx, a red mineral. Priestley called the gas he discovered dephlogisticated air , but this name would not stick.
In , Antoine Lavoisier , a French scientist, conducted many experiments with dephlogisticated air and theorized that the gas made some substances acidic. He renamed Priestley's gas oxygen , from the Greek words that loosely translate as 'acid maker'. While Lavoisier's theory about oxygen and acids proved incorrect, his name stuck. Lavoisier knew from other scientists before him that acids react with some metals to release another strange and highly flammable gas called phlogiston.
Lavoisier mixed the two gases, phlogiston and the newly renamed oxygen, in a closed glass container and inserted a match. He saw that phlogiston immediately burned in the presence of oxygen, and afterwards he observed droplets of water on the glass container. After careful testing, Lavoisier realized that the water was formed by the reaction of phlogiston and oxygen, and so he renamed phlogiston hydrogen , from the Greek words for 'water maker'.
Lavoisier also burned other substances such as phosphorus and sulfur in air, and showed that they combined with air to make new materials.
These new materials weighed more than the original substances, and Lavoisier showed that the weight gained by the new materials was lost from the air in which the substances were burned.
From these observations , Lavoisier established the Law of Conservation of Mass , which says that mass is not lost or gained during a chemical reaction. Elements are used up when they fuel chemical reactions, so resulting substances have less mass.
The main sign that we have terrain yet to be explored is the presence of a "singularity," or a point of infinite density, at the beginning of the Big Bang. Taken at face value, this tells us that at one point, the universe was crammed into an infinitely tiny, infinitely dense point.
This is obviously absurd, and what it really tells us is that we need new physics to solve this problem — our current toolkit just isn't good enough.
Related: 8 ways you can see Einstein's theory of relativity in real life. To save the day we need some new physics, something that is capable of handling gravity and the other forces, combined, at ultrahigh energies.
And that's exactly what string theory claims to be: a model of physics that is capable of handling gravity and the other forces, combined, at ultrahigh energies. Which means that string theory claims it can explain the earliest moments of the universe. One of the earliest string theory notions is the "ekpyrotic" universe, which comes from the Greek word for "conflagration," or fire.
In this scenario, what we know as the Big Bang was sparked by something else happening before it — the Big Bang was not a beginning, but one part of a larger process. Extending the ekpyrotic concept has led to a theory, again motivated by string theory, called cyclic cosmology.
I suppose that, technically, the idea of the universe continually repeating itself is thousands of years old and predates physics, but string theory gave the idea firm mathematical grounding. The cyclic universe goes about exactly as you might imagine, continually bouncing between big bangs and big crunches, potentially for eternity back in time and for eternity into the future.
As cool as this sounds, early versions of the cyclic model had difficulty matching observations — which is a major deal when you're trying to do science and not just telling stories around the campfire.
The main hurdle was agreeing with our observations of the cosmic microwave background, the fossil light leftover from when the universe was only , years old. While we can't see directly past that wall of light, if you start theoretically tinkering with the physics of the infant cosmos, you affect that afterglow light pattern.
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