Cosmologists study the universe at a level where records vanish, as we learn in this article: Best Time to Study the Cosmos Was More Than 13 Billion Years Ago. The physical world, with its stars and galaxies and background radiation fields, doesn’t leave us some sort of permanent record to study at our leisure. Information can disappear in various ways.
New calculations by Harvard theorist Avi Loeb show that the ideal time to study the cosmos was more than 13 billion years ago, just about 500 million years after the Big Bang. The farther into the future you go from that time, the more information you lose about the early universe.
“I’m glad to be a cosmologist at a cosmic time when we can still recover some of the clues about how the universe started,” Loeb said.
Two competing processes define the best time to observe the cosmos. In the young universe the cosmic horizon is closer to you, so you see less. As the universe ages, you can see more of it because there’s been time for light from more distant regions to travel to you. However, in the older and more evolved universe, matter has collapsed to make gravitationally bound objects. This “muddies the waters” of the cosmic pond, because you lose memory of initial conditions on small scales. The two effects counter each other — the first grows better as the second grows worse.
Loeb asked the question: When were viewing conditions optimal? He found that the best time to study cosmic perturbations was only 500 million years after the Big Bang.
This is also the era when the first stars and galaxies began to form. The timing is not coincidental. Since information about the early universe is lost when the first galaxies are made, the best time to view cosmic perturbations is right when stars began to form.
The information about the more raw levels of thing-like being disappear as it develops into larger and more complex structures. When gas clouds collapse to form stars and galaxies, the information about those clouds disappear.
As much as you might know about the entities and events at an advanced stage of a narrative, a story, you can’t always form a good idea of the earlier stages of the story. Anyone who’s read many biographies will know that there are those who pass through crisis points which don’t allow predictions of where that character will be heading when he’s past that cusp. If you start at the maturity of that character, you don’t always have a clue what his childhood and young adulthood was like. This is true of a simple function which has an immediate change in direction, such as y=absolute_value(x), which has a discontinuity at 0, that is, there is no unique tangent. It’s also true of those who pass through an important moral decision of certain sorts. For example, Adolf Hitler started life as a rather ordinary fellow and made a decision, or likely a series of decisions, which led him to a politics of hatred and exclusion. He headed off in a radically different direction from the prior trajectory of his life. There are entities, in the history of men and in the phystory of our universe, which have stable development paths. And there are periods where most entities have such paths. Take an ordinary American born in the period 1940-1970 or so and, with high probability, his general path through life, his political and business and charitable and religious activities, could be predicted. The general cluelessness of most such Americans in the event of dramatic changes in circumstances could have been predicted by those who knew history at a grander scale. What about those born into a society which had first stopped moving forward in significant ways, even in the physical prosperity so beloved by most men, and then passed through various crisis periods? A flattening and then erratic movements in various directions.
Various structures, from the moral character of individuals to that of various levels of communities, have formed in ways and under conditions which couldn’t have been predicted by following the trajectory of events in the 40s and 50s and even the 60s though there were strong hints of coming troubles in that latter decade and weaker hints for the more knowledgeable and more sensitive in the earlier two decades.
If you have no recorded information of structures and the circumstances and the states of individual entities before a crisis point — a discontinuity, it’s hard, and sometimes impossible, to recover that information.
Discontinuities. Nonlinearities. Cusps. There are many ideas explored by modern mathematicians and physicists which were well known under other terms by insightful historians and poets back when science was passing through its own Garden of Innocence when the universe seemed to behave in ways that seemed to be more rational. Functions describing physical reality were smooth, infinitely differentiable, well-behaved in most important ways. A scientist so clear-head as Laplace could convince himself the future of the universe is fully predictable so long as you have a complete knowledge of the current state of that universe.
In fact, the world is chaotic in the scientific sense of being well-determined, though perhaps not fully determined, but having an unpredictable future. I’ve discussed this before from a slightly different viewpoint, but one closely related to that of the study discussed in the article I referred to above: Best Time to Study the Cosmos Was More Than 13 Billion Years Ago.
Though the universe is moving through some sort of, as yet poorly understood, space of states, there is a time-symmetry to many physical process that leads to the strange situation of the past being as `unpredictable’ as the future, in some ways and for certain ranges of time in the past.
As I noted in my first published book, To See a World in a Grain of Sand (see To See a World in a Grain of Sand for a description of this book):
There’s a pretty good way to generate physical events which are chaotic by some realistic and practical standard. The measurements of those events will be a stream of numbers random by some standard, sometimes quite high. You can simply put two independent systems in contact with each other and observe or measure what happens at the interface. That’s all. It’s a trick used in some of the simple experiments used to generate so-called chaotic motion. For example, take two pendulums with different periods of oscillation. Link the bobs of the pendulums and put one or both in motion. The two pendulums will clearly not be able to move as they would if each moved freely. In fact, the resulting motion will be chaotic, basically unpredictable. Equivalent experiments can be done with electronic components or even with simulations of the independent systems in software. There’s nothing mystical involved in generating streams of physical events which are unpredictable or chaotic, generating numbers which are random by some low standard.
Chaotic motion can be visualized as an orbital path that never quite returns to the same point and is unpredictable beyond some length of time in the future or in the past. If you were to graph the orbits of such an object, and the orbiting earth is such an object, you would get a blur of orbits that lie close to each other and cross over often but no particular orbit is the same as any other. That sort of movement is patternless to human perception and measurement though usually staying within some tight boundaries. Physicists have shown by way of demanding computer simulations that “the orbital movement of planets in the solar system is chaotic… which makes practically impossible any precise prediction beyond a few tens of millions of years…”
The early results of Professor [Jacques] Laskar’s research (his first simulations of the solar system [were carried out] in 1989) indicate that a mere 15 meter error in measurement of the earth’s current position makes it impossible to say if the earth’s orbit will be stable 100 million years from now. Because the equations of dynamics are symmetrical in time, this means it’s also impossible to prove the earth’s orbit was stable more than 100 million years in the past, with that range of ignorance moving with us so far as the future goes and also moving with us so far as the past goes. A few years later, Gerald Sussman and Jack Wisdom of MIT showed that after only 4 million years it is not possible to predict the orbit of the earth, or any of the planets of the solar system, with any confidence.
Undoubtedly, results will have been tightened up in the technical literature to which I have no access. But the principle is what’s important.
In a sense, knowledge is constantly coming into view ahead of us and constantly disappearing behind us. To be more explicit, this means that, despite common sense, we cannot prove by mathematical means that the earth stayed in its orbit 4 million years ago. So far as the equations go, with initial conditions provided by the current state of the solar system, the earth might have crashed into the sun or gone shooting past Mars 4 million years ago. Common sense sometimes tells us things that mathematics cannot.
You shouldn’t imagine that this situation occurs only at the level of planets like the earth. The sun itself is traveling a chaotic orbit around the center of the galaxy. Our galaxy, the Milky Way, is dancing around various gravitational centers of local clusters of galaxies and larger-scale clusters of galaxies.
[In that book, I go on to a discussion that confuses predictability and determination, but the real issue is knowledge and predictability, chains of causality are sometimes unknown to us, whether well-determined or a bit looser than that.]