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How old is the universe?

With the adoption of the heliocentric model the size of the universe increased enormously: the very small value of stellar parallax implies that the universe is very big.

But how old is it? It may be eternal, but can we at least say that it is older than some particular age? It is definitely older that 123 years, CU-Boulder exists for so long.

Geological evidence suggested that older rocks were at the bottom, and newer rocks were at the top. The vast amount of rocks then implied that the Earth was very old, but no one new how much old.

Within rocks there were fossils of extinct animals $\longrightarrow$ the idea of animal evolution.

Jean-Baptiste Lamarck: transformation theory. Animals do change over time, in order to climb the hierarchical ladder of perfection, with the humans on top.

Charles Darwin (1809-1882), also Alfred Russel Wallace (1823-1913): theory of natural selection. There is no any predetermined ladder, animals change in order to survive. Humans are just one kind of animals. They differ from other animals because ... \framebox{\Huge\bf ?}

A:
humans can speak;
B:
humans can count;
C:
humans use tools;
D:
humans attend universities;
E:
humans wonder at the night sky;
F:
none of the above;

If all creatures gradually change through a process of natural selection, then the Earth could not be young, at least several billion years.

\framebox{\Huge\bf ?}William Thompson (later became Lord Kelvin) and Hermann Helmholtz independently computed the age of the Sun to be about 100 million years, too short for geological and biological evolution. There were no mistake in their calculation, they assumed that the Sun shined because it was contracting and heating up.

A:
Darwin and Wallace are wrong, there is no evolution and the Earth is very young.
B:
The earth is old, but the Sun is young, it formed after the Earth.
C:
Kelvin and Helmholtz missed something.

Discovery of nuclear physics fixed the problem. Using radioactive dating, we now know that:

Also, using models of stellar evolution, we can infer that the age of the oldest stars is about 13 billion years. This is not a direct measurement, since it relies on models of stellar evolution, which, albeit deemed correct for the vast majority of astronomers, can still contain an error.

In the Big Bang cosmology the universe is only a tiny bit older than the oldest stars (perhaps, by 200 million years).

BUILDING PHYSICAL COSMOLOGY

Cosmology is the science about the universe. In this context the word ``universe'' means the physical universe, the universe of material objects, of energy, of space and time. The main proposition of science is that the universe can be understood in terms of natural laws. The origin of these laws is not a subject of modern science, but may become a subject of science some day in the future.

Scientists call ``physical'' anything that can be objectively observed (at least in principle).

Space and time are also physical. Cosmology thus studies space and time as well as material objects, energy, radiation, etc.

\framebox{\Huge\bf ?}What do you think about questions like:

Your book says these questions are meaningless. Do you agree?

A related question is whether the universe was created, or existed ``forever''? Can an ``existing forever'' universe be created?

No matter how curious and fascinating those questions are, science does not address them. Those questions are ``unscientific''. By the definition of the physical universe, there can never be made an observation which will help to resolve those questions.

Science, however, can address a question of initial conditions, i.e. how the universe looked some time after the ``creation''. This relates to scientific questions like:

Even if these questions are scientific, we have no good answers to them now.