During a
solar eclipse in 1919, Arthur Eddington observed light bending around the sun
just as predicted by general relativity, Albert Einstein’s new theory of
gravity. Since then, general relativity, which says that massive objects like
stars warp the fabric of space-time around them, has passed increasingly
precise tests. A year rarely goes by without a new experiment or observation
confirming Einstein’s theory. But there’s a hitch.
Invisible
substances known as dark matter and dark energy seem to make up some 95% of the
content of the universe. The working assumption is that dark matter consists of
nonluminous elementary particles, and that dark energy is the energy of space
itself. But it’s also possible that they are illusions that appear because
gravity works differently from how Einstein thought about it. “We’re invoking
these mysterious things,” said the cosmologist Celia Escamilla-Rivera.“I am
strongly convinced that alternative theories of gravity are needed.”
Escamilla-Rivera
is searching for another, more complete theory. A bewildering array of
alternatives to general relativity have been put forward over time, from
“teleparallel gravity” to “complex quintessence” and “negative-mass cosmology,”
but they long seemed like theoretical fancies. With cosmologists unable to
create experiments that can distinguish these theories from general relativity,
the ideas have gathered dust.
According
to Escamilla-Rivera, that’s beginning to change in this new era of precision
cosmology, a field she is pioneering in her home country of Mexico. Precision
cosmology combines large and diverse data sets with new statistical methods,
machine learning and supercomputers. “Thanks to this data you can open a door
and classify all these theories and say which ones work and which ones don’t,”
she said.
By
scouring the early universe and the extreme environments of black holes,
Escamilla-Rivera thinks we can find cracks in general relativity, which will
make way for something else. This isn’t conventional wisdom among cosmologists,
but Escamilla-Rivera’s path to becoming a cosmologist hasn’t been conventional
either.
Escamilla-Rivera
grew up in Ciudad del Carmen, a city on a small island in southern Mexico. She
remembers walking along the beach at night under a full moon at age 4,
wondering: Why is the moon very round? And why does it only come out at night?
“I thought I needed to become an astronaut,” she said.
Years
later, watching one of her university professors compute the age of the
universe directly from Einstein’s equations, she switched her focus to
cosmology. “People were like: ‘Why do you want to be a cosmologist? These are
careers for people in the United States,’” she said. “It was seen as very
weird.”
Escamilla-Rivera
completed her doctorate in Europe and the United Kingdom; then, at 29, she was
invited back to Mexico to run the theoretical physics department of the
Mesoamerican Center for Theoretical Physics. A few years later, she became the
first woman to hold a research position in the department of gravitation and
field theory at the National Autonomous University of Mexico (UNAM) — as the
head of the department.
We spoke
for four hours over Zoom. From her office at UNAM in Mexico City,
Escamilla-Rivera exuded confidence and enthusiasm, not only about the potential
of precision cosmology to overturn Einstein, but also about new prospects for
cosmologists in Mexico. The interview has been condensed and edited for
clarity.
The
problem is that general relativity is not general enough. If you want to
explain dark energy, this invisible energy that seems to be accelerating the
universe’s expansion, you need an extra component in the equation, called the
cosmological constant. This extra component doesn’t exist naturally in general
relativity; you need to add it by hand.
Is there
a theory that naturally can give you dark energy without invoking mysterious
things? That’s why I’m working on these extended or modified theories of
gravity.
General
relativity explains a lot about nature, but it doesn’t explain what happened at
the Big Bang, or what happens inside black holes. The singularity of a black
hole is mathematically very similar to the Big Bang singularity — it’s a point
where all the known laws of physics break. A big question is, if we modify or
extend general relativity, maybe we can explain this bizarre point that breaks
everything.
General
relativity also doesn’t explain the future of the universe. There are
interesting theories that say the universe is going to collapse again into
another Big Bang, called the Big Crunch universe. But we don’t know because
general relativity is incomplete. If we find a complete theory, we could get
answers to these kinds of questions.
Reference:
0 Comments