Astronomy

infinite universe

Open, Flat, or Closed Universe: What Curvature Tells Us

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Direct answer: Cosmologists classify the universe as open, flat, or closed based on spatial curvature. Current cosmic microwave background (CMB) measurements…
white dwarfs

Why There Are Maximum Mass Limits for Compact Objects

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In this article, we will look at why there are maximum mass limits for objects that are supported against gravity by degeneracy pressure instead of kinetic…
dark energy supernova

Dark Energy Part 3: Fitting the SCP Union 2.1 Supernova Data

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In Part 1 of this 3-part series, I explained kinematics in Einstien-deSitter (EdS) cosmology and in Part 2, I explained kinematics in ##\Lambda##CDM cosmology…
dark energy cosmology lcdm

Dark Energy Part 2: LCDM Cosmology

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This is Part 2 of a 3-part series explaining evidence for so-called “dark energy” leading to a current positive cosmological acceleration. The evidence…
supernova refsdal

Supernova Refsdal: Multiply-Imaged Supernova Guide

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Supernova (SN) Refsdal — Key PointsAstronomers have been trying to understand space for thousands of years. Supernova Refsdal was the first known…
Dark Energy Cosmology Part 1

Dark Energy Part 1: Einstein-deSitter Cosmology

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In this 3-part series, I want to motivate the (re)introduction of the cosmological constant ##\Lambda## into Einstein's equations of general relativity…
Evidence for Big Bang

Big Bang Evidence: CMB, Redshift & Element Abundances

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Big Bang: Key PointsHubble's law (redshift of galaxies) and the Doppler effect provide direct evidence that the universe is expanding away from our…
Cosmic Web Connections

Cosmic Web Connectivity: How It Shapes Galaxy Evolution

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Introduction The universe was not perfectly uniform at the beginning; some regions had higher density than others. Over time these higher-density regions…
Gaia search for black holes

Gaia Astrometry: Detecting Gravitational Waves Guide

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Overview Gravitational waves (GWs) are disturbances in spacetime produced by massive objects moving asymmetrically. Only the most massive and most relativistic…
uranus spin

Why Uranus Spins on Its Side: Tilt, Theories, Models

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Uranus' Peculiar Tilt Uranus spins on its side. It has an obliquity (tilt) of 98° — its rotation axis lies closer to the ecliptic plane than any other…
black holes are not empty

VHE Gamma Rays from Sgr A*: Black Holes and CTA Observations

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Researchers simulating the accretion flow around Sagittarius A*, the supermassive black hole at the Milky Way's center, have found that magnetic reconnection…
Solar Balmer Lines

Signature of the Sun: Analyzing the Solar Balmer Series Lines

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Introduction We are in many ways a fortunate generation having so many wonderful tools at our disposal on account of the "silicon revolution". The advent…
isotropy definition

A Formal Definition of Large-Scale Isotropy

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This Insight is part of my attempt to develop a formal definition of 'large-scale isotropy', a concept that is fundamental to most cosmology, but that…
Telescope Buying Guide

Beginner Telescope Buying Guide: Dobsonians, Binoculars & More

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A beginner telescope buyer should start with 7x50 binoculars and star charts, then join a local astronomy club to try different telescopes before buying.…
Sagittarius A

Calculate Sgr A* Spin from QPOs — Kerr Metric Derivation

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Introduction This Insight explains how it is possible to calculate the spin of Sagittarius A* — the supermassive black hole at the center of the Milky…
sun_getting_smaller

MESSENGER Detects Sun’s Mass Loss and Solar Expansion

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Paper and context Paper discussion: Solar system expansion and strong equivalence principle as seen by the NASA MESSENGER mission. Antonio Genova, Erwan…
bb_infinity2

Big Bang, Infinity, and the Universe’s Shape Explained

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The Big Bang was not an explosion from a point in space. It was the appearance of a singularity, a moment where spacetime itself breaks down and cannot…
journeycosmos3

Learn About the FLRW Metric and The Friedmann Equation

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Previous Chapter: A Journey Into the Cosmos – The Friedmann Equation                Chapter 2- FLRW Metric and The Friedmann Equation In…
expandinguniverse

Explore Coordinate Dependent Statements in an Expanding Universe

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In this Insight, we will discuss the general Robertson-Walker (RW) universe, in which a set of co-moving observers with proper time ##t## observe the universe…
cosmosintro

Learn About the Friedmann Equation and the Cosmos

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This is an introduction to cosmology for someone who has some knowledge of calculus and basic physics. In this tutorial, we will take a journey into the…
solarimaging

Solar System Imaging: Telescope, Camera & Filters Guide

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Introduction Some of the most visible features in our sky are solar system objects: the Sun, the Moon, and the planets. These objects are generally much…

15 of the Best PF 2017 Eclipse Photos

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The 2017 August Eclipse really was something special. Fortunately Physics Forums has great photographers and snapped some remarkable shots. Here are a…
inflation5

A Poor Man’s CMB Primer: Quantum Seeds

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 The CMB establishes a record of ancient acoustic oscillations in the baryon-photon plasma. We've been studying how these primordial sound…
astro_advanced

Astrophotography: Mounts, Tracking & Image Processing

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Series linksPart 1: Introduction to Astrophotography Part 2: Intermediate Astrophotography Part 3: Advanced AstrophotographyMotivation "I’ve…
astro_inter

Astrophotography Guide: Equipment, Exposure & Processing

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Complete Series Part 1: Introduction Astrophotography Part 2: Intermediate Astrophotography Part 3: Advanced Astrophotography Level…
astro_intro

Astrophotography Guide: Beginner to Advanced Tips & Gear

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Part 1: Introduction to Astrophotography Part 2: Intermediate Astrophotography Part 3: Advanced AstrophotographySeries & prerequisites For…
cmbprimer4

A Poor Man’s CMB Primer: Cosmic Acoustics

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 Before decoupling, photons and charged particles were in good thermal contact: though the primordial plasma might have varied in temperature…
universestart

Did the Big Bang Have a Center? The Physics Explained

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The Big Bang did not happen at a single point in space, so it has no center. Standard cosmological models based on general relativity treat the Big Bang…
nightsky

Why Night-Sky Stars Often Appear Brighter Than the Sun

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Are most visible stars brighter than the Sun? A couple of weeks back, a thread was started about whether or not most of the stars we see at night are…
universeexpand

Big Bang Models: Expansion, Dark Energy, Inflation

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Big Bang and expansion (no inflation, no dark energy) Empty-universe solution (FLRW) Einstein's General Relativity allows a solution (the FLRW metric)…
lightcone7_3

LightCone 8 Tutorial Part III – How Things are Computed

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In Part I and Part II of this mini-series, we have briefly discussed the basic user interface and the use of charts to depict the LCDM cosmological model.…
universeexpansion

At What Velocity Does the Universe Expand and Can It Be Faster than Light?

Neither of these questions makes sense in the form in which it was posed. To see why, let's start by thinking about how we know the universe is expanding.The…
darkmatter

Knut Lundmark and the Forgotten Dark Matter Discovery

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Background: textbook accounts Look in your standard cosmology textbook (in my case, the closest one at hand is Barbara Ryden's Introduction to Cosmology)…
lightcone72

LightCone8 Tutorial Part II – Charts

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Part I dealt with the basic user interface of LightCone8. This part of the tutorial is about potentially useful cosmological insights to be gained from…
quasarredshift

Are Galaxy Redshifts Cosmological? The Evidence Explained

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Galaxy and quasar redshifts are classified as cosmological, not "intrinsic," because they match predictions from general relativity's expanding-universe…
lightcone7

LightCone8 Tutorial Part I

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LightCone 8 is a versatile tabulating/charting cosmological calculator, useful for understanding the expansion history of the universe (and even some future…
earlyuniverse

Why Wasn’t the Early Universe at Maximum Entropy?

No. The second law of thermodynamics states that entropy can only increase over time. If the early universe had begun in a state of maximum entropy, the…
observableuniverse

Why the Observable Universe Is Bigger Than Its Age

The observable universe has a radius of about 46 billion light-years, far larger than its age of roughly 14 billion years. This happens because space itself…
steadystate

What Is the Steady-state Model and Why It’s No Longer Viable

Introduction to the Steady State Model Back in 2011, I wrote a FAQ entry for Physics Forums on the steady state model and why it is no longer viable.…
blackholesuniverse

Why the Big Bang Wasn’t a Black Hole: Cosmology Explained

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The Big Bang was not a black hole because it happened everywhere in space at once rather than at a single point, and because the near-perfect homogeneity…
universespinning

Is the Universe Rotating? What Physics Tests Reveal

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All current astronomical observations are consistent with zero global rotation of the universe. Solar-system data set a model-independent upper limit of…
blackbody

A Poor Man’s CMB Primer: Bumps on a Blackbody

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 Astronomers Arno Penzias and Robert Wilson discovered the cosmic microwave background in 1965. They were not looking for it.They were…
birthcmb

A Poor Man’s CMB Primer: The Birth of a Cosmic Background Radiation

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 The early universe was hot. So hot that nuclei boiled. The great thermal energy of the universe overwhelmed the confining efforts of the…
balloonanalogy

The Balloon Analogy Explained: Cosmic Expansion Without a Center

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The balloon analogy uses pennies glued to an inflating balloon to show two ideas from modern cosmology: that unbound regions of the universe expand roughly…
cmb

A Poor Man’s CMB Primer: Orientation of the Universe

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 This is a picture of the cosmic microwave background:Fig 1. The cosmic microwave background as seen by the European Space Agency's…
CosmologicalHorizons

Inflationary Misconceptions and the Basics of Cosmological Horizons

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Introduction It is a common saying that during inflation "space expanded faster than the speed of light."  This statement is meant to articulate the…
matterphoton

Will All Matter Be Converted to Photons?

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Will all matter be converted into black holes and then photons? No. There is a misconception that a lot of laypeople seem to be picking up recently from…
LCDM4

Approximate LCDM Expansion in Simplified Math (Part 4)

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  Part 4: Cosmic Recession Rates An astronomer, accompanied by his amateur relativist friend, aimed a telescope at a distant galaxy and measured…
LCDM3

The LCDM Cosmological Model in Simplified Math (Part 3)

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  Part 3: Important Cosmological Horizons and Distances A question that often comes up is: "how big is the observable universe?"The question…
lcdmjorrie

Approximate LCDM Expansion in Simplified Math (Part 2)

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  Part 2: Time and Distance In Part 1 we have seen how one of the simplest ways of expressing the fractional expansion rate of the universe,…
cosmo

Approximate LCDM Expansion in Simplified Math

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 If we restrict ourselves to a spatially flat LCDM universe model (and ignore the early radiation energy density), the first Friedmann equation…
earth year

How to Scientifically Measure How Many Days Are in a Year

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How many days are there in a year? Ask pretty much anyone and they will tell you 365, except for leaps years which have 366. In addition,…
solar_eclipse

Solar Eclipse Viewing & Safety Guide: What to Expect

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A solar eclipse is safe to view with the naked eye only during totality, the brief period when the Moon completely covers the Sun. At every other stage…
supernova

Star Death Explained: White Dwarfs to Black Holes

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Stellar death outcomes depend on the star's initial mass: stars below roughly 8 solar masses end as white dwarfs, while more massive stars undergo core…