Where Did the Universe’s Energy Originally Come From?
I have always found one question about the universe especially fascinating. If energy cannot be created or destroyed, where did the universe's energy come from in the first place?
The honest answer is that scientists do not know for sure. What I find even more interesting is that the familiar rule about energy becomes more complicated when we try to apply it to the entire expanding universe.[5]
Energy Changes Form
In everyday life, energy can move from one place to another and change form. When I eat food, my body uses its chemical energy to move, stay warm, and keep my organs working.
A car turns energy stored in fuel into motion and heat. A battery powers a phone. In each case, the energy does not simply appear from nowhere. It comes from somewhere and changes form.
This everyday rule is reliable, but I think it is important to remember that a car or a human body is not a closed system. Energy enters and leaves both of them.
Much of Our Food Energy Comes From the Sun
Plants use sunlight to make sugars through photosynthesis. When animals eat plants, or other animals that ate plants, they get some of that stored chemical energy. Much of the energy in my meals can be traced back to sunlight.
There are exceptions. Some ecosystems, such as communities around deep-sea hydrothermal vents, rely on chemical energy rather than sunlight.[9] Nature does not always follow the same path.
The Sun Turns Mass Into Energy
Deep inside the Sun, nuclear fusion combines hydrogen nuclei into helium. The resulting helium has slightly less mass than the starting material, and the difference is released as energy. Some of that energy eventually reaches Earth as sunlight.[6]
Einstein's famous equation expresses the connection:
E = mc²
Here, E is energy, m is mass, and c is the speed of light. Because the speed of light squared is a huge number, a small amount of mass corresponds to a large amount of energy. In fusion, the Sun loses a little mass as energy is carried away.
Not Everything Comes From Today's Sun
Sunlight powers much of life on Earth, but other energy sources follow different paths:
- Nuclear energy: Uranium contains energy that can be released through nuclear fission. Heavy elements such as uranium are thought to form in extreme cosmic events, including neutron star mergers.[4]
- Geothermal energy: Earth's interior contains heat left over from its formation, as well as heat produced by radioactive decay.[7]
- Tidal energy: The Moon's gravity, along with the Sun's, helps produce tides. Tidal power draws on energy from the Earth-Moon system's motion.[8]
To me, these examples show how many different forms and histories energy can have. They also raise the question of where the story began.
Hydrogen and the Early Universe
The hydrogen that fuels the Sun was not made by the Sun. Much of it traces back to the early universe.[1]
In the first few minutes after the Big Bang, the universe cooled enough for protons and neutrons to form the earliest atomic nuclei, mostly hydrogen and helium. It was still too hot for those nuclei to hold on to electrons. Complete atoms formed much later, about 380,000 years after the Big Bang.[1]
That hydrogen later became part of stars, planets, water, and living things. I find it remarkable that some of the hydrogen in our bodies has such an ancient history.
Does Energy Conservation Apply to the Whole Universe?
Here is where things become more subtle. In physics, the familiar conservation of energy is closely connected to a symmetry: the laws describing a system do not change when an experiment is shifted in time. But the expanding universe is not a static background.
General relativity still has a local conservation law for energy and momentum. What becomes difficult is defining one conserved total energy for the entire expanding universe, including gravity. That is different from saying that anything can happen or that local energy conservation has stopped working.[5]
Light gives us a useful example. As light travels across expanding space, its wavelength stretches. This is called cosmological redshift. The more the wavelength stretches, the lower the energy of each photon measured by observers moving with the cosmic expansion.[3][5]
So where does the missing energy go? In an expanding universe, there is not generally a simple, universally agreed cosmic energy account into which it must be deposited. I find that surprising, but it does not mean that the physics has broken down.[5]
Dark Energy Adds Another Mystery
Observations indicate that the universe's expansion is accelerating. Scientists use the name dark energy for whatever is responsible in the standard cosmological picture, but they do not yet know its nature.[2]
One possibility is a cosmological constant, an energy associated with space whose density stays constant. If that model is correct, the amount of dark energy in an expanding region grows as the region's volume grows. That does not violate local conservation in general relativity, even though it challenges our everyday intuition about a fixed total.[5]
Dark energy is not the same thing as dark matter. I explored the latter in Could We Be Close to Finding Dark Matter?, another question where the evidence is compelling but the underlying nature remains unknown.
Could the Universe Have Started With Zero Energy?
One idea I find intriguing is that positive energy in matter and radiation could be balanced by negative gravitational energy, leaving a total of zero.
It is an interesting proposal, not an established answer. In general relativity, gravitational energy and the total energy of an expanding universe are not always defined in a simple, unique way. A zero balance would also not, by itself, explain how or why the universe began.[5]
Measurements suggest that space is very close to flat on large scales, but that observation does not prove that the universe has zero total energy or that it appeared from nothing.[5]
The Big Question Remains
I can trace much of the energy in my food back to plants and sunlight, and I can follow the Sun's hydrogen back to the early universe. But that journey does not give me a confirmed answer to the deepest question: why was there an energetic universe at all?
Perhaps a future theory will reveal an earlier history, or show that the question needs to be asked differently. For now, I think the honest answer is also the most exciting one: we are still learning.
What do you think is the most surprising part of the universe's energy story?
Related Posts
Sources
[1] https://science.nasa.gov/universe/overview [2] https://science.nasa.gov/dark-energy [3] https://www.einstein-online.info/en/redshift [4] https://science.nasa.gov/image-article/apod-2017-october-16-gw170817-a-spectacular-multiradiation-merger-event-detected [5] https://preposterousuniverse.com/cosmologyprimer/faq.html [6] https://science.nasa.gov/sun/facts [7] https://www.energy.gov/hgeo/geothermal/geothermal-faqs [8] https://www.noaa.gov/education/resource-collections/ocean-coasts/tides [9] https://www.pmel.noaa.gov/eoi/nemo/explorer/concepts/chemosynthesis.html