Physics and Cosmology
Theories on the origin and nature of the universe.
Where Does a Redshifted Photon’s Energy Go?
A redshifted photon has less energy at reception. A worked wavelength example separates local conservation from global energy accounting in an expanding universe.
Why Does Electron-Positron Annihilation Produce a 511-keV Line?
The 511-keV line reflects electron and positron rest energy. Two-photon annihilation, positronium, and motion explain the line and its limits.
Why Does IceCube Look Through Earth for Neutrinos?
Earth can suppress atmospheric muons while allowing neutrinos through. IceCube’s indirect light measurements still require energy-dependent background checks.
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How Much Does the Sun’s Gravity Bend Starlight?
The Sun’s gravity bends starlight by a small, measurable angle. A worked example distinguishes distance from the center, apparent position, and measurement limits.
How Can Galaxies Recede Faster Than Light?
Galaxies recede faster than light in a cosmological sense: their separation grows. Learn why this differs from local motion and what it means for visibility.
Why Don’t Most Cosmic Rays Point Back to Their Sources?
Cosmic rays bend in magnetic fields. Understand rigidity, energy and charge, then see why a measured arrival direction may not identify one source.
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Why Can Distant Gamma Rays Turn Into Electron-Positron Pairs?
Distant gamma rays can interact with background photons to create electron-positron pairs. Energy, collision angle and probability govern attenuation.
Why Is the Innermost Stable Orbit Different From a Black Hole’s Horizon?
The innermost stable orbit concerns orbital stability; an event horizon concerns escape. Their radii differ, and spin changes the simple comparison.
How S301’s Orbit Could Reveal Sagittarius A*’s Spin
S301’s orbit offers a way to test how a rotating black hole changes stellar motion. Learn which observations could reveal spin and which effects can confuse the measurement.
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