Privacy

Open Cosmology has no accounts, no cookies, no advertising and no tracking or visitor statistics. What it remembers for you stays in your own browser.

What is stored on your device

These are kept in your browser's storage for this site. They are never sent to the site or to anyone else.

• Your study progress: which lessons you marked as studied, your best quiz scores and the last lesson you opened (for “Continue where you left off”).
• Your reading options: text size, spacing, font, contrast, line length, link highlighting and animations.
• Read-aloud settings: the voice, speed and similar choices.
• Light or dark mode, if you chose one with the theme button.
• Offline copies: courses you saved for offline use, and copies of the last 60 pages you opened (they were downloaded anyway, and let you reread them without a connection). A small background script (a service worker) serves these when you are offline and keeps saved copies up to date.
• For the current browser session only: whether you dismissed the “new version available” notice.

Removing it

• Study progress: “Clear my progress” on the home page.
• Reading options: “Reset all” in Reading options (the “Aa” button).
• Offline copies: “Remove offline copies” in Reading options.
• Everything at once: clear this site's data in your browser's settings.

Nothing loaded from other sites

Fonts, formulas, graphs, search and the PhET simulations are all served from this site itself, so opening a page does not contact any other company. Links to other sites (MIT OpenCourseWare, PhET, arXiv, GitHub and others) are only followed when you click them; those sites then have their own privacy policies.

Read-aloud voices

Read-aloud uses the voices of your own browser and device. Most run on the device, but some online voices, such as the “Google” voices in Chrome and the “Online” voices in Microsoft Edge, send the text being read to Google or Microsoft to produce the sound. If you prefer that no text leaves your device, choose a voice without “Google” or “Online” in its name in the read-aloud settings.

Hosting

The site is hosted by Vercel. Like every web host, Vercel's servers record technical information about each request — such as the IP address, the page requested, the time and the browser — to deliver the site and protect it against abuse. Open Cosmology does not use these records to identify or follow visitors. See Vercel's privacy policy for how it handles them.

Reporting mistakes and other feedback

Mistakes and suggestions are collected as issues on GitHub. Issues there are public and need a GitHub account; GitHub's own privacy policy applies. Please do not put personal information in an issue.

Changes

If the site ever starts storing or sending anything else — for example cookie-free visitor statistics — this page will say so first. Last updated 15 November 2026.