Astronomy

infinite universe

Open, Flat & Closed Universes: Curvature Explained

/
0 Comments
Standard cosmological models are classified by spatial curvature into three broad types: open, flat, and closed universes. These correspond to negative,…
white dwarfs

Why There Are Maximum Mass Limits for Compact Objects

/
0 Comments
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

/
0 Comments
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

/
0 Comments
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

/
0 Comments
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

/
6 Comments
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

/
2 Comments
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

/
1 Comment
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

/
1 Comment
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

/
4 Comments
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

/
9 Comments
Black Hole Key PointsBlack holes (BHs) have a gravitational pull so strong that nothing, not even light, can escape. BHs can be divided into two…
Solar Balmer Lines

Signature of the Sun: Analyzing the Solar Balmer Series Lines

/
2 Comments
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

/
4 Comments
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 Guide: Choose the Right First Scope

/
141 Comments
What telescope should a beginner buy? Overview for beginners People often ask what telescope an aspiring amateur astronomer should buy. There isn't a single…
Sagittarius A

Calculate Sgr A* Spin from QPOs — Kerr Metric Derivation

/
3 Comments
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

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

Intro to the Big Bang and Infinity Concepts

/
34 Comments
In the cosmology section of the Physics Forums, I encountered many Big Bang and related infinity problems. Hence, in this article, I want to explain the…
journeycosmos3

Learn About the FLRW Metric and The Friedmann Equation

/
1 Comment
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

/
4 Comments
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

/
6 Comments
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

/
4 Comments
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

/
69 Comments
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

/
11 Comments
 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

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

Astrophotography Guide: Equipment, Exposure & Processing

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

Astrophotography Guide: Beginner to Advanced Tips & Gear

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

A Poor Man’s CMB Primer: Cosmic Acoustics

/
1 Comment
 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? Explaining Cosmology

/
28 Comments
Did the Big Bang Have a Center? What the standard models say According to standard cosmological models based on general relativity — and which agree…
nightsky

Why Night-Sky Stars Often Appear Brighter Than the Sun

/
3 Comments
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

/
29 Comments
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

/
1 Comment
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

/
16 Comments
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

/
3 Comments
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

Evidence that Galaxy and Quasar Redshifts Are Cosmological

/
0 Comments
How do we know that the redshifts of galaxies and quasars are cosmological and not "intrinsic"? Evidence for the cosmological interpretation Historical…
lightcone7

LightCone8 Tutorial Part I

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

Was the Early Universe in a Disordered State?

No. The second law of thermodynamics says that entropy can only increase, so if the early universe had been in a state of maximum entropy, then the cosmos…
observableuniverse

Why the Observable Universe Radius Exceeds Its Age

The radius of the observable universe is about 46 billion light-years, which is considerably greater than its age of about 14 billion years. The radius…
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 Early Big Bang Universe Didn’t Form a Black Hole

/
7 Comments
Why the Big Bang Is Not a Black Hole In the early universe, the matter was gathered together at very high density — so why wasn't it a black hole?Big…
universespinning

Is the Universe Rotating? Evidence, Tests & Limits

/
3 Comments
Can we tell whether the universe is rotating? If you believe wholeheartedly in Mach's principle, there is no way to test empirically for the rotation…
blackbody

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

/
15 Comments
 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

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

Balloon Analogy Explained: Cosmology, Expansion & Limits

/
62 Comments
The Balloon Analogy is a straightforward way to illustrate — without being complete — two important points of modern cosmology that many people find…
cmb

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

/
4 Comments
 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

/
45 Comments
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?

/
41 Comments
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)

/
14 Comments
  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)

/
5 Comments
  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)

/
14 Comments
  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

/
11 Comments
 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

/
75 Comments
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

/
0 Comments
Eclipse Viewing Key PointsNever view the Sun with the naked eye for any length of time; doing so can cause irreversible eye damage or blindness. …
supernova

Star Fate: From Nebulae to White Dwarfs and Black Holes

/
0 Comments
Star Fate: From Birth to Black Holes Stars form in vast clouds of gas and dust called nebulae. Over time gravity pulls material together until the core…