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- The recent passing of the great theoretical physicist Steven Weinberg reminded me of how his book led me to study cosmology.
- Going back in time to cosmic childhood is a spectacular effort that combines experimental and theoretical ingenuity. Modern cosmology is an experimental science.
- Cosmic history is ultimately ours. Our roots go back to the first moments after creation.
When I was a junior in college, my electromagnetism teacher had a brilliant idea. In addition to the usual homework and exams, we were to give a class seminar on a topic of our choice. The idea was to assess which area of physics we would be interested in pursuing professionally.
Professor Gilson Carneiro knew I was interested in cosmology and suggested a book by Nobel Prize winner Steven Weinberg: The first three minutes: a modern view of the origin of the universe. I still have my original Portuguese copy from 1979, which has a musty tropical smell, sitting on my shelf next to the American version, a Bantam edition from 1979.
Inspired by Steven Weinberg
Books can change lives. They can light the way to go. In my case, there is no doubt that Weinberg’s book blew my teenage mind away. I decided, then and there, that I would become a cosmologist working on the physics of the early universe. The first three minutes of cosmic existence – what could be more exciting for a young physicist than trying to uncover the mystery of creation itself and the origin of the universe, matter and stars? Weinberg quickly became my hero of modern physics, the one I wanted to professionally emulate. Sadly, he passed away on July 23rd, leaving a great void for a generation of physicists.
What excited my young imagination was that science could actually make sense of the very first universe, which meant that theories could be validated and ideas could be tested against real data. Cosmology, as a science, didn’t really take off until after Einstein published his article on the shape of the universe in 1917, two years after his groundbreaking article on the theory of general relativity, the one explaining how we can interpret gravity as the curvature of space-time. . Matter does not “bend” over time, but it does affect the speed at which it flows. (See last week’s essay on What Happens When You Fall into a Black Hole).
The Big Bang Theory
For most 20e century, cosmology has lived in the realm of theoretical speculation. One model has proposed that the universe started from a small, hot, dense plasma billions of years ago and has grown steadily since – the Big Bang model; another suggested that the cosmos is stationary and that the changes observed by astronomers are mostly local – the steady state model.
Competing models are essential to science, but so is data to help us tell them apart. In the mid-1960s, a decisive discovery changed the game forever. Arno Penzias and Robert Wilson accidentally discovered microwave cosmic background radiation (CMB), a first universe fossil predicted by George Gamow, Ralph Alpher, and Robert Herman in their Big Bang model. (Alpher and Herman posted a nice tale of the story here.) CMB is a bath of microwave photons that permeates all of space, a holdover from the days when the first hydrogen atoms were forged, some 400,000 years after the bang.
The existence of the CMB was the smoking gun confirming the Big Bang model. From that moment on, a series of spectacular observatories and detectors, both terrestrial and space, extracted enormous amounts of information on the properties of CMB, much like the paleontologists who excavate the remains of dinosaurs and dig for more bones to collect. details of a bygone past.
How far can we go back?
Confirmation of the outline of the Big Bang model has changed our cosmic outlook. The universe, like you and me, has a history, a past to explore. How far can we go back in time? Was there an ultimate wall that we couldn’t cross?
Because matter gets hot as it is compressed, going back in time meant looking at matter and radiation at higher and higher temperatures. There is a simple relationship that connects the age of the universe and its temperature, measured in terms of the temperature of photons (particles of visible light and other forms of invisible radiation). The funny thing is that matter breaks down as the temperature rises. Thus, to go back in time is to look at matter in increasingly primitive states of organization. After the CMB formed 400,000 years after the bang, there were hydrogen atoms. Before, there weren’t any. The universe was filled with a primordial soup of particles: protons, neutrons, electrons, photons and neutrinos, the ghostly particles that traverse planets and people unharmed. In addition, there were very light atomic nuclei, such as deuterium and tritium (both heavier cousins of hydrogen), helium, and lithium.
Cosmic alchemy
So, to study the universe after 400,000 years, we have to use atomic physics, at least until large clumps of matter clump together under gravity and begin to collapse to form the first stars, a few million years later. And earlier? Cosmic history is broken down into chunks of time, each being the domain of different types of physics. Before atoms form, until about a second after the Big Bang, it’s nuclear physics time. This is why Weinberg brilliantly titled his book The first three minutes. It was during the interval between one hundredth of a second and three minutes that light atomic nuclei (made up of protons and neutrons) formed, a process called, with poetic flair, primordial nucleosynthesis. Protons collided with neutrons and sometimes stuck together due to the powerful and attractive nuclear force. Why then have only a few light nuclei formed? Because the expansion of the universe has made it difficult for particles to find each other.
What about the nuclei of heavier elements, like carbon, oxygen, calcium, gold? The answer is beautiful: all the elements of the periodic table after lithium were made and continue to be made in the stars, the true cosmic alchemists. Hydrogen eventually becomes human if you wait long enough. At least in this universe.
In this article, we went all the way to nucleosynthesis, the forging of the first atomic nuclei when the universe had one minute. And earlier? How far from the start, from t = 0, can science get closer? Stay tuned, and we’ll continue next week.
To Steven Weinberg, with gratitude, for everything you have taught us about the universe.
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