Cosmology

If the universe were infinite and unchanging, every line of sight would end on a star and the sky would blaze with light. This puzzle, Olbers' paradox, is resolved by the finite age of the universe.

Because the universe is not infinite in age, and its expansion plus light-travel limits mean much of the starlight has not reached us yet.

Because the universe is finite in age and the light from many distant stars has not had time to reach us. The expansion of the universe also stretches and dims light from very distant galaxies.

Spiral arms form because waves of higher density move through a galaxy’s disk, compressing gas and triggering new star formation. The arms are patterns, not rigid structures, and they can persist for hundreds of millions of years.

Elliptical galaxies are usually redder because they contain older stars and little cold gas for making new blue stars. Without much fresh star formation, their light is dominated by aging stellar populations.

Supermassive black holes likely grew over billions of years by swallowing gas, stars, and smaller black holes. Many galaxies also grew through mergers, which helped feed and enlarge their central black holes.

Quasars shine because matter falling into a supermassive black hole heats up to extreme temperatures before crossing the event horizon. The glowing accretion disk can release more light than billions of stars combined.

The cosmic microwave background is the leftover glow from when the universe first became transparent about 380,000 years after the Big Bang. It gives us a snapshot of the infant universe and helps measure its age, composition, and shape.

Before that, the universe was a hot plasma that scattered light constantly. Once it cooled enough for electrons to bind with protons, photons could travel freely through space.

Space itself is stretching, so distant galaxies are carried apart even if they are not moving through space like thrown objects. This expansion was first measured by Edwin Hubble in the 1920s.

We can only see light that has had time to reach us since the beginning of the universe. Because the universe is about 13.8 billion years old and has expanded, the observable universe is much larger than 13.8 billion light-years across.

Stars form in giant molecular clouds because those clouds are cold, dense, and rich in molecular hydrogen and dust. Gravity can then collapse clumps of gas into protostars.

Massive stars burn through fuel much faster because their cores are hotter and under greater pressure. Small stars use fuel slowly, so they can shine for tens of billions of years.

Red dwarfs often flare because their magnetic fields are very active and tangled. Their convection zones can generate powerful magnetic storms that release bursts of radiation.

A star’s final fate depends mainly on its mass. Stars like the Sun shed their outer layers and leave behind a white dwarf, while much heavier stars collapse further into neutron stars or black holes.

Neutron stars spin fast because they inherit the original star’s rotation, and their collapse shrinks them dramatically. Conservation of angular momentum makes a tiny remnant rotate much faster, sometimes hundreds of times per second.

Pulsars flash because their magnetic poles are misaligned with their spin axis, so their radiation sweeps past us like a lighthouse beam. Each rotation produces a pulse if Earth lies in the beam path.

Galaxies are mostly empty space, so stars are extremely far apart compared with their sizes. During a collision, gravity reshapes the galaxies, but direct star-to-star impacts are very unlikely.

The Milky Way became a disk because rotating gas flattened as it cooled, conserving angular momentum. Over time, stars formed in that flattened rotating structure.

Visible matter alone does not provide enough gravity to explain how galaxies and clusters behave. Dark matter helps account for galaxy rotation, cluster motions, and the large-scale structure of the universe.

Dark energy is the name given to whatever is driving the accelerated expansion of the universe. Observations of distant supernovae in the late 1990s showed that expansion is speeding up, not slowing down.

Light is redshifted because the universe expands while the light is traveling, stretching its wavelength. That stretching makes distant objects appear redder than they really are.

Mass bends spacetime, and light follows that curved path. A massive galaxy cluster can act like a lens, magnifying and distorting galaxies behind it.

Jets are launched by magnetic fields near rapidly feeding black holes. Rotating plasma in the accretion disk and black hole environment can be channeled into narrow beams moving near light speed.

Bars form when gravity rearranges the orbits of stars and gas in a disk galaxy. The bar can funnel gas toward the center and help fuel new star formation or black hole activity.

The Milky Way’s center formed and grew along with the galaxy, allowing a black hole seed to accrete matter over billions of years. Sagittarius A* now contains about 4 million solar masses.

The first stars formed from nearly pure hydrogen and helium, with almost no heavy elements. Without those cooling elements, they were probably much more massive and short-lived than most stars today.

The Moon blocks the Sun’s bright surface, or photosphere, allowing the faint corona to become visible. The corona is normally hidden by glare because it is far dimmer than the Sun’s disk.

Solar heat vaporizes ice in the comet, releasing gas and dust. Solar wind and radiation push that material away from the nucleus, forming tails.

Because the universe has a finite age and is expanding, most starlight has not had time to reach us, and much of it is stretched into invisible wavelengths.

As space itself expands, the wavelength of light traveling through it stretches, shifting it toward the red end of the spectrum.

It is leftover radiation from the early universe, released about 380,000 years after the Big Bang, and it now fills all of space.

Galactic centers are where matter tends to gather over time, and repeated mergers and gas inflows help grow a central black hole. These black holes can reach millions or billions of solar masses.

Quasars are powered by supermassive black holes pulling in huge amounts of gas, which heats up and radiates enormous energy. The accretion disk can outshine the stars in its host galaxy.

Observations show that cosmic expansion is accelerating, likely because of dark energy, a mysterious component that makes up about 68% of the Universe. Its effect becomes more important as space itself expands.

The cosmic microwave background is leftover radiation from the early Universe, emitted when it first became transparent about 380,000 years after the Big Bang. Because the early Universe was hot and dense, that light is remarkably uniform across the sky.

Galaxies and clusters rotate and hold together as if they contain much more mass than visible matter alone can explain. That missing mass is attributed to dark matter.

When very massive objects accelerate violently, they disturb spacetime and send out ripples called gravitational waves. The strongest signals come from compact objects spiraling together and colliding.

Looking far away means looking back in time, because light takes time to travel. Distant regions show the Universe when it was younger, and galaxies were forming rapidly.

Tiny density fluctuations in the young Universe grew under gravity, and dark matter helped pull gas into the first structures. Once gas cooled, the first stars and galaxies could form surprisingly early.

Stars fuse light elements into heavier ones, and exploding stars spread those elements into space. Later generations of stars and planets formed from that enriched material.

Space itself is stretching, a process first recognized from galaxy redshifts and later explained by modern cosmology.

Galaxies contain huge numbers of stars and move through space under gravity, so over billions of years their paths often cross. Individual stars rarely collide, but the galaxies themselves strongly interact and merge.

The universe is finite in age, so light from the most distant stars has not had enough time to reach us. Also, expansion stretches and dims distant light.

The early universe was hot and opaque, but as it expanded and cooled, that radiation was stretched into microwaves. It is the afterglow of the Big Bang.

Before neutral atoms formed, free electrons scattered light constantly, making the universe opaque. Once atoms formed, light could travel much more freely.

Distant supernova observations showed in 1998 that cosmic expansion is accelerating, likely because of dark energy. Dark energy seems to act like a property of space itself.

Dark matter is inferred because visible matter alone cannot explain how galaxies and galaxy clusters move. Its gravity appears to add extra mass that we cannot see directly.