How do we know that Dark Energy has homogeneous energy density?
Is the energy density of vacuum uniform in the whole universe?
Printer service not found on port 515 For services list on my Ubuntu, Printer service not found on port 515.  nmap -sT -O localhost  PORT     STATE SERVICE 23/tcp   open  telnet 139/tcp  open  netbios-ssn 445/tcp  open  microsoft-ds 631/tcp  open  ipp 9050/tcp open  tor-socks Device type: general purpose Running: Linux 3.X OS CPE: cpe:/o:linux:linux_kernel:3 OS details: Linux 3.12 - 3.19, Linux 3.8 - 3.19       There is no Printer service on port 515. How to add Printer service on port 515?     nmap -v -p 515 127.0.0.1  Starting Nmap 7.01 ( https://nmap.org ) at 2016-11-19 13:03 IRST Initiating SYN Stealth Scan at 13:03 Scanning localhost (127.0.0.1) [1 port] Completed SYN Stealth Scan at 13:03, 0.21s elapsed (1 total ports) Nmap scan report for localhost (127.0.0.1) Host is up (0.000065s latency). PORT    STATE  SERVICE 515/tcp closed printer Challenges with Key with Gaps? Produce an explicit bijection between rationals and naturals? What happens to matter in a standard model with zero Higgs VEV? Suppose you reset the parameters of the standard model so that the Higgs field average value is zero in the vacuum, what would happen to standard matter?  If the fundamental fermions go from a finite to a zero rest mass, I'm pretty sure that the electrons would fly away from nuclei at the speed of light, leaving positively charged nuclei trying to get away from each other.  Looking at the solution for the Hydrogen atom, I don't see how it would be possible to have atoms with zero rest mass electrons.  What happens to protons and neutrons?  Since only a very small part of the mass of protons and neutrons is the rest mass of the quarks, and since they're flying around in there at relativistic speeds already, and since the nuclear force is so much stronger than the electrical force with an incredible aversion to naked color, would protons and neutrons remain bound assemblages of quarks and virtual gluons?  Would they get a little larger?  A little less massive?  What would happen to nuclei?  Would they stay together?  If the protons and neutrons hold together and their properties change only some, then I might expect the same of nuclei.  Different stable isotopes, different sizes, and different masses, but I would expect there would still be nuclei.  Also the W and Z particles go to zero rest mass.  What does that do to the electroweak interactions?  Does that affect normal stable matter (outside of nuclear decay modes)?  Is the weak force no longer weak?  What happens to the forces overall? What would I prefer - an over-fitted model or a less accurate model? Let's say we have two models trained. And let's say we are looking for good accuracy.  The first has an accuracy of 100% on training set and 84% on test set. Clearly over-fitted. The second has an accuracy of 83% on training set and 83% on test set.   On the one hand, model #1 is over-fitted but on the other hand it still yields better performance on an unseen test set than the good general model in #2.   Which model would you choose to use in production? The First or the Second and why? Scoping in assigning a derivative Are real guns used in movies? Conditional page breaking I have used LaTeX on and off for a number of years now for several large documents with evolving versions over long lifetimes. But every time it seems like, powerful and wonderful as LaTeX is, there arise cases where I want to tweak the pagination in ways that are impossible without using the \newpage or \clearpage sledgehammer.  The macro I think I want is something like  \ifnotremaining{&lt;length&gt;}{&lt;cmd&gt;}   that is, if less than  space remains on the page then perform . For example:  \ifnotremaining{5cm}{\newpage}   Note that &lt;length&gt; need not be perfect–if a little too big no problem, or a little small (perhaps because the "protected" text got bigger) then you just increase it a bit. FWIW, I am usually happy with \raggedbottom.  An alternative might be a \disablepagebreak and \enablepagebreak pair which could be used to forbid a page break in the given range of text. (This capability cannot be formulated as an environment because it cannot always nest wrt other environemnts.)  Is there something I have overlooked that can fill this need? Problem  sudo notify-send Test "Hello World"   Displays a notification as expected.  notify-send Test "Hello World"   Does not display a notification.  Further information  Ubuntu version 16.04.  The notifications appear to use notify-osd instead of notification-daemon. Running notify-send appears to launch a notify-osd process under the user that ran notify-send. I'm not sure what dbus is.   No error messages  There are no errors in the syslog. When I run the following code no error messages occur.   #include &lt;libnotify/notify.h&gt; #include &lt;stdio.h&gt; int main() {     gboolean x = notify_init ("Hello world!");     printf( "notify_init: %d\n", x );     NotifyNotification * Hello = notify_notification_new ("Hello world", "This is an example notification.", "dialog-information");     GError *err = NULL;     x = notify_notification_show (Hello, &amp;err);     printf( "notify_notification_show: %d\n", x );     if(err != NULL) {         printf("Error detected!\n");         printf("Error message:%s\n", err-&gt;message);     }     else {         printf("No error detected.\n");     }     g_object_unref(G_OBJECT(Hello));     notify_uninit();     return 0; }   I do not know what I'm doing. Thanks for the help.  Python Notify  In the past, I've used a python program that has working notifications. I tested a hello world notification and it works without sudo!   #!/usr/bin/env python # -*- coding: utf-8 -*- import gi gi.require_version('Notify', '0.7') from gi.repository import Notify  Notify.init("test") n = Notify.Notification.new('test', 'test2') n.set_urgency(Notify.Urgency.CRITICAL)     n.show()   Code from   I am not sure why this works. The python Notify is just wrapping the library used in the C example. I'd prefer not to use python but will if I must.  Edit  The problem occurred again. Resolved by adding urgency critical. Is there a platform-agnostic and filesystem-agnostic method to obtain the full path of the directory from where a program is running using C/C++? Not to be confused with the current working directory. (Please don't suggest libraries unless they're standard ones like clib or STL.)  (If there's no platform/filesystem-agnostic method, suggestions that work in Windows and Linux for specific filesystems are welcome too.) $AB-BA$ is a nilpotent matrix if it commutes with $A$ I saw this in a  and am putting it here for posterity.  Problem: Let $A$ and $B$ by square matrices and set $C=AB-BA$. If $AC=CA$, prove $C$ is nilpotent. Smaug first appeared in TA 2770 as a mature Fire-drake. I'm not sure if there are canonical sources which say his age or maturity, but he must have been quite large and quite old at the time in order to destroy the city of Dale and capture Erebor.  Since it is safe to assume that Smaug would have been attacking towns and stealing all of the gold and jewels he could get his claws on for most of his life, then he must have amassed a huge fortune prior to taking residence at Erebor. How did he transport his existing horde to Erebor? It seems unlikely that he would have simply abandoned all of his treasure that he had previously accumulated.  EDIT: It has been established that Smaug was not a mature drake at the time of his appearance. However, he may have already had a small horde (compared to Erebor). Crick’s Central Dogma — Counter Cases How do I inspect the view hierarchy in iOS? Single supply op amp to convert 0-5VDC to +/-2VDC (microcontroller to line level) I am generating a 0-5 volt sine wave using an arduino with a DAC. I would like to level shift this signal (DC bias?) by -2.5 volts and have a gain of .8, so that I end up with a line level sine wave.  Thus I am trying to design a single supply op amp with the following equation:  $$V_{\text{o}} = (V_{\text{i}}-2.5) \times 0.8$$  I found this white paper that is supposed to describe a way to do this but I'm having trouble following as I am new to op-amps.    Does anyone know of a way to do this with a single op amp or IC?  The frequency range of 20Hz to 20KHz must be preserved. Can I carry spare Lithium-Ion batteries with me on Lufthansa? I am travelling from the US to India on Lufthansa. I have five spare Lithium-ion batteries, each 850mAh.  Can I bring them back safely? If I want to make a backup of everything I have done since the fresh install of Ubuntu, what are the possible options? What all should I backup? I want to get all the settings that I changed, all the packages I installed, etc. A and B disjoint, A compact, and B closed implies there is positive distance between both sets A white number inside a black circle. How to make a white number inside a black circle (the picture below)? I want to change numbers inside the black circle, of course.  I use Share LaTeX. I have read some articles here but anyone are not what I would like to get. I would like to get this  (the picture below) - the sixth answer.   Thanks for help. What is the meaning of the axes in t-SNE? I'm currently trying to wrap my head around the  math. Unfortunately, there is still one question I can't answer satisfactorily: What is the actual meaning of the axes in a t-SNE graph? If I were to give a presentation on this topic or include it in any publication: How would I label the axes appropriately?  P.S: I read  Reddit question but the answers given there (such as "it depends on interpretation and domain knowledge"), don't really help me to understand this.