What is dark energy?
Even though gravity is pulling inward on space-time — Cosmos keeps expanding outward faster and faster. To account for this, Scientists have proposed an invisible agent that counteracts gravity by pushing space-time apart. They call it dark energy. In the most widely accepted model of dark energy, it is a “cosmological constant”: an inherent property of space itself, which has “negative pressure” driving space apart. As space expands, more space is created, and with it, more dark energy.
What is Dark Matter?
About 84 percent of the matter in the universe does not absorb or emit light. “Dark matter,” as it is called, cannot be seen directly, and it hasn’t yet been detected by indirect means, either. Instead, dark matter’s existence and properties are inferred from its gravitational effects on visible matter, radiation and the structure of the universe.
The quest to find the missing universe is one of the key efforts that has brought cosmologists and particle physicists together. The leading dark-matter candidates are neutrinos or two other kinds of particles: neutralinos and axions, predicted by some physics theories but never detected. All three of these particles are thought to be electrically neutral, thus unable to absorb or reflect light, yet stable enough to have survived from the earliest moments after the Big Bang
Does nature have more than four spacetime dimensions? If so, what is their size? Are dimensions a fundamental property of the universe or an emergent result of other physical laws? Can we experimentally “see” evidence of higher spatial dimensions?
There are two kinds of forces that operate over two different scales—gravity for big scales like galaxies, the other three forces for the tiny world of atoms . As per unified theory proponents—there must be a way to connect the three atomic-scale forces with gravity. Maybe, but it won’t be easy. In the first place, gravity is odd. Einstein’s general theory of relativity says gravity isn’t so much a force as it is an inherent property of space and time. Accordingly, Earth orbits the sun not because it is attracted by gravity but because it has been caught in a big dimple in space-time caused by the sun and spins around inside this dimple like a fast-moving marble caught in a large bowl. Second, gravity, as far as we have been able to detect, is a continuous phenomenon, whereas all the other forces of nature come in discrete packets.
All this leads us to the string theorists and their explanation for gravity, which includes other dimensions. The original string-theory model of the universe combines gravity with the other three forces in a complex 11-dimensional world
Where do ultrahigh-energy particles come from?
The most energetic particles that strike us from space, which include neutrinos as well as gamma-ray photons and various other bits of subatomic shrapnel, are called cosmic rays. They bombard Earth all the time. Cosmic rays are sometimes so energetic, they must be born in cosmic accelerators fueled by cataclysms of staggering proportions. Scientists suspect some sources: the Big Bang itself, shock waves from supernovas collapsing into black holes, and matter accelerated as it is sucked into massive black holes at the centers of galaxies. Knowing where these particles originate and how they attain such colossal energies will help us understand how these violent objects operate.
Is string theory correct?
When physicists assume all the elementary particles are actually one-dimensional loops, or “strings,” each of which vibrates at a different frequency, physics gets much easier. String theory allows physicists to reconcile the laws governing particles, called quantum mechanics, with the laws governing space-time, called general relativity, and to unify the four fundamental forces of nature into a single framework. But the problem is, string theory can only work in a universe with 10 or 11 dimensions: three large spatial ones, six or seven compacted spatial ones, and a time (race against time phrase meaning) dimension. The compacted spatial dimensions — as well as the vibrating strings themselves — are about a billionth of a trillionth of the size of an atomic nucleus. There’s no conceivable way to detect anything that small, and so there’s no known way to experimentally validate or invalidate string theory
How did the universe begin?
If all four forces of nature are really a single force that takes on different complexions at temperatures below several million degrees, then the unimaginably hot and dense universe that existed at the Big Bang must have been a place where distinctions between gravity, strong force, particles, and antiparticles had no meaning. Einstein’s theories of matter and space-time, which depend upon more familiar benchmarks, cannot explain what caused the hot primordial pinpoint of the universe to inflate into the universe we see today. We don’t even know why the universe is full of matter. According to current physics ideas, energy in the early universe should have produced an equal mix of matter and antimatter, which would later annihilate each other. Some mysterious and very helpful mechanism tipped the scales in favor of matter, leaving enough to produce galaxies full of stars